It's generally said that discoveries in science tend to be at the thin hairy edge of what you can do -- always at the faintest limits you can see, the furthest reaches, the lowest signals. That can be trivially true because stuff that's easy to find has already been discovered. But many times, when you're looking farther and fainter than you ever have, you find things that really are new... and can (maybe!) be a problem for existing models of how the Universe behaves. Astronomers ran across just such thing recently. Hubble observations of a distant galaxy cluster revealed an arc of light above it. That's actually the distorted image of a more distant galaxy, and it's a common enough sight near foreground clusters. But the thing is, that galaxy shouldn't be there.
This picture is a combination of two images taken in the near-infrared using Hubble. The cluster is ...
