...when scientists are wrong. The exciting things happen when we realize our understanding of the universe is not what we thought it was.
And the latest one is a doozy. The University of Colorado Boulder put a giant spectograph on the Hubble. The Cosmic Origins Spectograph has been analyzing the stuff in outer space. (Which is not empty).
They've been looking at traces of hydrogen floating between galaxies, because we can do cool stuff like that now. When hydrogen is struck by UV light, it becomes ionized - charged. Here's the problem.
There are more hydrogen ions in intergalactic space than can be explained by the existing sources of UV light. Oops. And not just a few more. A lot more. 80% more. Maybe we're wrong about how hydrogen behaves when that far from a gravity well? The lead theory is that the ionization is somehow being caused by the decay of dark matter.
Oh, and then it gets really interesting. This effect is only being seen relatively close to us. When we look further away, the amount of ionization is perfectly explained by the number of quasars and very young stars - the UV sources we know about.
So, whatever is causing this, it's new. By the standards of the universe, anyway. Decay of dark matter could indeed explain it, but in that case something happened to speed it up.
What else could be new? New kinds of stars, we'd know about. And as much as I'd love to jump on the theory, starship exhausts wouldn't create that much energy, surely...
So, we have a mystery. And the only time our understanding of the universe can evolve is when we're wrong about something.