They were probably bombarding us with messages a few million years ago, but unfortunately they went way over the heads of the dinosaurs. I bet the aliens were very disappointed, thinking "surely there's got to be intelligent life out there, hasn't there? Why aren't we hearing anything back?".
I'm not saying that I don't believe that there is the posibility that life has existed elsewhere in the universe at some point since the big bang. But given the, in cosmic terms, incredibly small amount of time we have had the ability to detect any possible signs of extra-terestrial life I think it is highly unlikely that we will ever actually detect any such signs. To do so would involve those signs being given off at absolutely the right moment for them to reach us between the first radio telescope being built in 1937 and now - 78 in 13.8 billion are not great odds...
They may have been bombarding us with messages 200 years ago, or for that matter every 200 years for the past billion years, and we were never in a position to notice - let alone respond.
We are simply unequipped to determine the odds that we will receive an alien communication, likewise on why we have have received nothing so far. Way too many variables.
On a more serious note might pop along to this Stunning astronomy images set to inspire awe at Southampton exhibition (From Daily Echo)
I've been following this last week... all I can say is... "WAAAAAAAAAH!" :Wideyed: Have We Really Discovered A Huge Alien Megastructure Around A Star? | IFLScience