Concept
Dark Matter
The invisible mass holding galaxies together.
Makes up about 27% of the universe. It does not emit, absorb, or reflect light, but its gravity bends starlight and keeps galaxies from flying apart.
See also
Reionization Epoch
When the first stars and galaxies relit the universe.
Hubble Flow
The general expansion-driven motion of galaxies.
Cosmic Expansion
Space itself stretching, carrying galaxies apart.
Dark Matter Halo
A galaxy's enveloping cloud of invisible matter.
The Big Bang
How everything began, 13.8 billion years ago.
Black Holes
Regions where gravity stops light itself.