Fermi paradoxIf the universe is vast and old, the silence from other civilizations feels strange.10 min / Easy 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 Black hole information paradoxA conflict between quantum theory and black holes asks whether information can vanish.11 min / Deep 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 Hubble Deep FieldA long stare into empty-looking sky revealed thousands of distant galaxies.7 min / Easy 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 Search for extraterrestrial intelligenceSETI listens for signs that intelligence elsewhere has become detectable.9 min / Easy Arecibo messageA radio message sent toward a star cluster became a symbol of cosmic outreach.7 min / Easy Voyager Golden RecordHumanity sent sounds and images into interstellar space aboard Voyager.7 min / Easy Pale Blue DotA distant photograph of Earth became a meditation on perspective.6 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 Kessler syndromeCollisions in orbit could create enough debris to trigger further collisions and restrict access to space.8 min / Medium 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 Berserker hypothesisA darker answer to the Fermi paradox imagines self-replicating probes eliminating emerging civilizations.8 min / Medium 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 Black dwarfTheoretical stellar remnants would emerge only after white dwarfs cool for timescales far beyond the universe's current age.8 min / Medium