system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do we know the universe is expanding and what causes it?
gpt
The reason why we know it is expanding is because the galaxies farther away from us are red shifted. Which means the wavelength of light is shifted to the red part of the spectrum. It is similar to the a doppler effect when a sound is moving away from you. 

We know that part of the reason why galaxies are moving away is because spacetime is expanding. The redshifted galaxies led to the big bang theory. If the galaxies at far distances are moving away from each other now then at one point they were probably much closer than before. 

The acceleration of the expansion of the universe is what we do not understand yet. We know that it is happening because of the redshift of supernovae. We don't know what is causing the acceleration so we assigned the name of dark energy to it until we can identify it.

Dark matter is the unknown matter that is keeping individual galaxies together.