It’s known as the Wow! signal. It was discovered during a SETI project—i.e., they were trying to find intelligent aliens. There have been a bunch of debates as to what the hell the Wow! signal actually is, but no real consensus.
There is a non-zero probability that the Wow! signal came from an extraterrestrial intelligence; however, given the strength of the signal and how far it must have traveled, whatever civilization sent it would have to be way, way more advanced than we are. Any messages we could send with the most powerful radio transmitters we have would degrade into static long before they would get to where the Wow! signal may have originated.