We now know this is due to cosmic expansion. Space itself is expanding, which makes distant galaxies appear to recede away from us. The rate of this expansion is known as the Hubble parameter, and while we have a good idea of its value, there is still a bit of tension between different results.

One of the difficulties in resolving this tension is that thus far we can only measure cosmic expansion as it appears right now. This also means we can't determine whether cosmic expansion is due to general relativity or a more subtle extension of Einstein's model. But as powerful new telescopes are built, we might be able to observe the evolution of cosmic expansion thanks to what is known as the redshift drift effect.

The Hubble parameter has a value of about 70 km/s per megaparsec. This means if a galaxy is about 1 megaparsec away (about 3 million light-years), then the galaxy appears to be moving away from us at about 70 km/s. If a galaxy is 2 megaparsecs away, it will appear to recede at about 140 km/s. The greater a galaxy's distance, the greater its apparent speed.

Since the universe is still expanding, with each passing year a galaxy is a bit more distant, and that means its redshift should become slightly larger. In other words, cosmic expansion means that the redshifts of galaxies should drift more to the red over time.