Black holeA black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping.3 min / Deep Fermi paradoxThe Fermi paradox is the seeming inconsistency between the lack of evidence of extraterrestrial civilizations and the apparently high likelihood of their existence.3 min / Deep Space: ExoplanetAn exoplanet or extrasolar planet is a planet outside the Solar System.3 min / Deep James Webb Space TelescopeThe James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy.3 min / Deep Kessler syndromeThe Kessler syndrome, also known as the Kessler effect, collisional cascading, or ablation cascade, is a scenario proposed by NASA scientists Donald J.3 min / Deep Space: MarsMars is the fourth planet from the Sun.3 min / Deep Space: 1I/ʻOumuamua1I/ʻOumuamua is the first confirmed interstellar object detected passing through the Solar System.3 min / Deep Pale Blue DotPale Blue Dot is a photograph of Earth taken on February 14, 1990, by the Voyager 1 space probe from an unprecedented distance of over 6 billion kilometers (3.7 billion miles, 40.5 AU), as part of that day's Family Portrait series of images of the Solar System.3 min / Deep Search for extraterrestrial intelligenceThe search for extraterrestrial intelligence (usually shortened as SETI) is the diverse efforts and scientific projects intended to detect extraterrestrial signals, or any evidence of intelligent life beyond Earth.3 min / Deep Voyager programThe Voyager program is an American scientific program that employs two interstellar probes, Voyager 1 and Voyager 2.3 min / Deep Black hole information paradoxThe black hole information paradox is an unsolved problem in physics and a paradox that appears when the predictions of quantum mechanics and general relativity are combined.3 min / Deep Hubble Deep FieldThe Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope.3 min / Deep Arecibo messageThe Arecibo message is an interstellar radio message carrying basic information about humanity and Earth that was sent to the globular cluster Messier 13 (M13) in 1974.3 min / Deep Berserker hypothesisThe Berserker hypothesis, also known as the deadly probes scenario, is the idea that humans have not yet detected intelligent alien life in the universe because it has been systematically destroyed by a series of lethal Von Neumann probes.3 min / Deep Black dwarfA black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently to no longer emit significant heat or light.3 min / Deep Chelyabinsk meteorThe Chelyabinsk meteor (Russian: Челябинский метеорит, romanised: Chelyabinskiy meteorit) was a superbolide that entered Earth's atmosphere over the southern Ural region in Russia on 15 February 2013 at about 09:20 YEKT (03:20 UTC).3 min / Deep Cosmic microwave backgroundThe cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe.3 min / Deep Dark flowIn astrophysics, dark flow is a controversial hypothesis to explain certain non-random measurements of peculiar velocity of galaxy clusters.3 min / Deep Dark forest hypothesisThe dark forest hypothesis is the idea that extraterrestrial civilizations may exist in abundance across the universe, but remain silent and hidden out of fear that revealing themselves would lead to destruction by a more technologically advanced and hostile civilization.3 min / Deep Drake equationThe Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.3 min / Deep Dyson sphereA Dyson sphere is a hypothetical megastructure that encompasses a star and captures a large percentage of its power output.3 min / Deep CMB cold spotThe CMB Cold Spot or WMAP Cold Spot is a region of the sky seen in microwaves that has been found to be unusually large and cold relative to the expected properties of the cosmic microwave background radiation (CMBR).3 min / Deep False vacuumIn quantum field theory, a false vacuum is a hypothetical vacuum state that is locally stable but does not occupy the most stable possible ground state.3 min / Deep Fast radio burstIn radio astronomy, a fast radio burst (FRB) is a transient radio wave of duration ranging from a fraction of a millisecond, for an ultra-fast radio burst, to 3 seconds, caused by a high-energy astrophysical process that is not yet understood.3 min / Deep Gamma-ray burstIn gamma-ray astronomy, gamma-ray bursts (GRBs) are extremely energetic events occurring in distant galaxies that represent the brightest and most powerful class of explosion in the Universe.3 min / Deep Great AttractorThe Great Attractor is a region of enhanced gravitational attraction in the local universe.3 min / Deep Great FilterThe Great Filter is the idea that in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there is a barrier to development that makes detectable extraterrestrial life exceedingly rare.3 min / Deep Heat death of the universeThe heat death of the universe (also known as the Big Chill or the Big Freeze) is a scientific hypothesis regarding the ultimate fate of the universe which posits the universe will evolve to a state of no thermodynamic free energy and, having reached maximum entropy, will therefore be unable to sustain any further thermodynamic processes.3 min / Deep Hoag's ObjectHoag's Object is an unusual ring galaxy in the constellation of Serpens Caput.3 min / Deep Hubble's lawHubble's law, officially the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance.3 min / Deep Space: K2-18bK2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth.3 min / Deep Kardashev scaleThe Kardashev scale (Russian: шкала Кардашёва, romanized: shkala Kardashova) is a method of measuring a civilization's level of technological advancement based on the amount of energy it is capable of harnessing and using.3 min / Deep Laniakea SuperclusterThe Laniakea Supercluster or Laniakea for short (; Hawaiian for "open skies" or "immense heaven") is the large-scale structure centered around the Great Attractor that is home to the Milky Way and approximately 100,000 other nearby galaxies.3 min / Deep Transient lunar phenomenonA transient lunar phenomenon (TLP) or lunar transient phenomenon (LTP) is a short-lived change in light, color or appearance on the surface of the Moon.3 min / Deep Space: MagnetarA magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G).3 min / Deep Neutron starA neutron star is the gravitationally collapsed core of a massive supergiant star.3 min / Deep Planet NinePlanet Nine is a hypothetical ninth planet in the outer region of the Solar System.3 min / Deep PSR B1919+21PSR B1919+21 is a pulsar with a period of 1.3373 seconds and a pulse width of 0.04 seconds.3 min / Medium Space: QuasarA quasar ( KWAY-zar) is an extremely luminous active galactic nucleus (AGN).3 min / Deep Great FilterA possible evolutionary or technological bottleneck in the search for alien life.9 min / Medium Heat death of the universeA far-future scenario where useful energy gradients disappear.9 min / Medium False vacuum decayA speculative cosmic instability with consequences that would outrun warning.8 min / Medium OumuamuaThe first known interstellar object passed through the Solar System and sparked debate.8 min / Easy Tabby's StarA star with strange dimming patterns invited hypotheses from dust to megastructures.8 min / Easy Planet NineA proposed distant planet that may explain unusual outer Solar System orbits.8 min / Easy Fast radio burstBrief, powerful radio flashes from deep space became a major astronomical puzzle.9 min / Medium Lorimer burstThe first detected fast radio burst opened a new field of investigation.7 min / Easy Rogue planetA planet drifting without a star turns familiar planetary life upside down.7 min / Easy Neutron starCollapsed stellar cores pack immense mass into city-sized objects.9 min / Medium MagnetarA neutron star with extreme magnetic fields can unleash staggering bursts.8 min / Medium QuasarBrilliant galactic cores reveal the power of supermassive black holes.8 min / Medium Gamma-ray burstThe universe's brightest explosions can outshine galaxies for moments.8 min / Medium Solar storm of 1859The Carrington Event showed how space weather can reach human technology.8 min / Easy Chelyabinsk meteorA meteor exploded over Russia, injuring people with its shock wave.7 min / Easy Theia (planet)A hypothesized ancient impactor may explain the origin of the Moon.8 min / Medium Bootes voidA vast cosmic region contains far fewer galaxies than expected.7 min / Easy Great AttractorGalaxies in our region appear to stream toward a massive hidden concentration.8 min / Medium Cosmic microwave backgroundAncient light from the early universe maps the afterglow of the Big Bang.10 min / Medium Kardashev scaleA scale ranks civilizations by the amount of energy they can use.7 min / Easy Dyson sphereA hypothetical megastructure captures a star's energy and fuels speculation.8 min / Easy Voyager Golden RecordHumanity sent sounds and images into interstellar space aboard Voyager.7 min / Easy Lunar transient phenomenonShort-lived changes seen on the Moon remain a tricky observational subject.8 min / Medium Dark forest hypothesisA chilling answer to cosmic silence imagines civilizations hiding from one another.8 min / Easy Drake equationA compact equation organizes the unknowns involved in estimating communicative civilizations in our galaxy.8 min / Easy Zoo hypothesisOne proposed answer to cosmic silence suggests advanced civilizations deliberately avoid contacting Earth.7 min / Easy PSR B1919+21The first discovered pulsar produced such regular signals that researchers briefly labeled it LGM-1.8 min / Easy Dark flowSome studies proposed that galaxy clusters share a vast motion not fully explained by standard cosmology.8 min / Medium Laniakea SuperclusterA map of galaxy motions placed the Milky Way within an immense basin of gravitational attraction.8 min / Medium Sloan Great WallA survey revealed an enormous filament of galaxies stretching across more than a billion light-years.8 min / Medium Eridanus SupervoidA vast underdense region of the universe has been examined for links to a cold spot in the cosmic background.9 min / Medium Hubble tensionDifferent methods for measuring cosmic expansion produce results that remain stubbornly inconsistent.10 min / Deep K2-18bA temperate exoplanet with a hydrogen-rich atmosphere has become a closely watched target in the search for habitability.8 min / Medium Thorne-Zytkow objectA hypothetical hybrid star would hide a neutron star inside the envelope of a red giant.8 min / Medium Hoag's ObjectA nearly perfect ring of young blue stars surrounds the older yellow core of an unusual galaxy.7 min / Easy