Big Bang Theory: The hypothesis that all matter and energy were at one time compresses into a very small volume; then there was an explosion that sent everything moving outward, causing the universe to expand.

Blueshift: Shift of wavelengths of light towards the blue end of the spectrum; happens as a light source moves toward us.

Galaxy: A collection of billions of stars.

Redshift: Shift of wavelengths of light towards the red end of the spectrum; happens as a light source moves away from us.

Universe: Everything that exists; all matter and energy; also includes all of space and time.