A mysterious substance that makes up most of the matter in the universe, but does not emit or absorb light.
A mysterious force that is accelerating the expansion of the universe, but its origin and nature are unknown.
The apparent contradiction between the high probability of the existence of extraterrestrial civilisations and the lack of evidence for, or contact with, such civilisations.
A discrepancy in the trajectory of the Pioneer 10 and 11 spacecraft that cannot be explained by known physics.
A strong radio signal detected in 1977 that has never been explained and remains a mystery to this day.
A massive explosion that occurred in 1908 over the remote Tunguska region of Russia, believed to have been caused by a comet or asteroid.
A region beyond Neptune where thousands of icy objects, including dwarf planets like Pluto, reside.
A region of space that is pulling the Milky Way and many other galaxies towards it, but its exact location and nature are unknown.
A hypothesized spherical cloud of icy objects believed to surround the outermost reaches of the solar system.
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