When I was in grad school, one of the hottest (ironically) topics of research was a phenomenon called “cooling flows.” When we look at clusters of galaxies — literally, collections of dozens or hundreds of galaxies like the Milky Way all orbiting one another, bound by their mutual gravity — we see lots of hot gas surrounding them. Over time, that gas should cool, flow down to the center, collect there, and form lots of stars.
The problem was, nobody could find those stars. Many of these clusters have galaxies right at their center, grown huge due to collisions with other galaxies that fell to the center. According to the numbers, though, so much cooling gas flows into them that they should be bright blue due to star formation — if you form enough stars, some percentage of them will be massive, hot, and blue, and those are so bright they overwhelm the light from redder stars. That’s why the Milky Way’s spiral arms look blue; it’s where star formation is going on.
But in cluster after cluster the central dominant galaxy wasn’t as blue as it should be. It was a big mystery. Were cooling flows real?