How We Know the Universe Is Expanding
The gist
We know the universe is expanding because we observe galaxies moving away from us and their light shifting toward the red end of the spectrum, called redshift.
Explain like I'm 5
It's like when you hear a siren getting lower in pitch as an ambulance drives away; the light from galaxies shifts in a way that shows they're moving away from us.
Big picture
This discovery supports the Big Bang theory and helps scientists understand the origin, age, and future of the universe.