The universe now is 13.8 billion years old. The color of a galaxy is an indicator of the age of the stars that populate it.

When we look at distant galaxies, we are looking back in time. We have now seen galaxies as they were when the universe was about 300 million years old—only about two percent as old as it is now.

color of a galaxy — an indicator of the age of the stars that populate it. cosmological principle — the assumption that, on the large scale, the universe at any given time is the same everywhere—isotropic and homogeneous. hot dark matter — massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation; hot dark matter is faster-moving material than cold dark matter. supercluster — a large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies.