Red Shift.

All galaxies appear to be moving away from each other. Over large distances, the further an object moves away from any given vantage point, the more the light waves emanating from it are stretched out before they reach it - resulting in lower and lower frequencies as it continues along its course. Within the spectrum of visible light, red light possesses the lowest frequency, meaning the further into the distance an object strays, the redder it appears from our slice of space and time.