Dark matter
Bent light and galaxy motions reveal gravity that known stars and gas cannot fully explain. What supplies the additional mass?
Cosmology
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Dark energy
The universe's expansion is accelerating. “Dark energy” names the gap in our explanation, while several different physical accounts remain possible.
Matter–antimatter asymmetry
Matter and antimatter can annihilate each other. Yet the observable universe contains enough leftover matter to make stars, planets, and people. What produced that imbalance?
Cosmic inflation
A short period of rapid expansion could explain several features of the early universe. What physical process, if any, produced it?
Hubble tension
Different ways of inferring the present expansion rate of the universe give results that do not fit comfortably together. Is the mismatch in the measurements, the model, or both?
Primordial lithium problem
Old, metal-poor stars contain less lithium than standard early-universe calculations predict. Did the stars destroy it, or is something missing from the calculation or inference?
Supermassive black-hole seeds
What initial black holes grew into the massive objects seen in the early Universe?
Origin of cosmic magnetic fields
Where did the seed magnetic fields amplified into cosmic magnetism come from?
Ionizing photon escape during reionization
How many ionizing photons escaped early galaxies and reached intergalactic hydrogen?