August 15th, 1977. The Big Ear Radio Telescope, located in the US, received a strong radio signal that lasted for 72 seconds. The signal was quite unusual, and the astronomers involved at the time weren't able to find a possible explanation behind its source. It was dubbed the "Wow! Signal".
As expected, whenever there's a mysterious extraterrestrial signal, the first hypothesis is always, "It must be aliens!". The publicity of the Wow! signal did play a role in making people wonder if intelligent life out there is trying to send us messages. It remains as the strongest candidate for radio transmissions from alien life to this day.
It turns out, the signal was most likely generated by a passing comet. However, we are not certain of it yet. Two comets, 266P/Christensen and P/2008 Y2 (Gibbs), were passing by that part of the sky when the signal was detected. There's a high possibility that one of the comets, which could have been accompanied by a hydrogen cloud, produced those radio signals. Details regarding this explanation can be found here: http://planetary-science.org/research/the-wow-signal/
Unfortunately, the craziest explanations are the ones that always get the most traction. "Alien civilizations" is always the first explanation that gets publicity, even when it's usually not even one of the top 10 candidates. This has happened too many times in recent times. Hoping for a wild theory to be true generally creates a bias that takes us away from finding the real answers.