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The observation that the Pioneer 10 and Pioneer 11 spacecraft were deviating from their predicted trajectories. Current explanations include the anisotropic emission of heat through the spacecraft's thermal radiation, which generates a faint, but detectable, recoil force.

A conflict between quantum mechanics and general relativity, suggesting that information could be lost in a black hole. Proposed resolutions include the holographic principle, black hole complementarity, or the possibility that information is encoded in Hawking radiation.

A strong narrowband radio signal detected by SETI in 1977 that matched expected characteristics of an interstellar signal and was never observed again. Explanations range from a signal of extraterrestrial origin to a passing comet or a human-made source.

A gravitational anomaly in intergalactic space that reveals the presence of a local gravitational dipole. The likely explanation is a massive, but distant, supercluster of galaxies, which is drawing the Milky Way and other nearby galaxies towards it.

A mysterious form of energy that is causing the acceleration of the expansion of the universe. The most popular explanation is the cosmological constant ($\Lambda$), a concept introduced by Einstein, which suggests a constant energy density filling space homogeneously.

An unseen substance that does not emit light or energy, inferred from gravitational effects on visible matter, radiation, and the structure of the universe. Theories include Weakly Interacting Massive Particles (WIMPs), axions, and modifications to Newtonian dynamics.

Brief, intense pulses of radio waves of unknown origin. Theories range from highly magnetized neutron stars and black hole activity to extraterrestrial intelligence. Some FRBs repeat, while others are one-off events.