Stephan's Quintet
Radio observations in the early 1970s showed a mysterious emission from the inter-galactic space between the galaxies in the group. This same region also showed faint glow of ionized atomic hydrogen. You can see this in the visible part of the spectrum as the magnificent green-blue arc in the picture to the right.  Two space telescopes have recently shown something about the nature of the strange filament. It is now believed to be a giant intergalactic shock-wave (similar to a sonic boom but travelling in intergalactic gas rather than air). This is caused by one galaxy (NGC 7318B) falling into the center of the group at several million miles per hour.