The observed universe
Unseen matter, cosmic expansion, energetic particles, and brief radio flashes.
40 problemsCosmology
The expansion, history, and large-scale structure of the universe.
Collections
Browse collectionsA few problems to explore
9 totalDark matter
Bent light and galaxy motions reveal gravity that known stars and gas cannot fully explain. What supplies the additional mass?
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?