RobertB
 There are thousands of galaxies like our own Milky Way.
 

Actually... there are hundreds of billions of galaxies (most with hundreds of billions of stars). Not all of them will be like ours but still... it seems more unlikely that we somehow are an anomaly in the universe. Especially considering they are now seeing that there are planets and moons within our own solar system that have appropriate conditions to support certain kinds of life.
 
That said the cosmic speed barrier (the speed of light) is what would prohibit any kind of meaningful interstellar travel. The only other ways would be if a species a) found a way to break that barrier b) engaged in MULTI generational expeditions or lived a VERY long time (and had a vessel that could survive that long/far), c) figured out how to manipulate physics in such a way as to bypass traditional truisms of the physical universe (like utilizing quantum entanglement to create copies of themselves elsewhere or being able to detect and reinforce and navigate wormholes or shift from dimension to dimension, etc...) or d) were some form of life that simply is not bound by physical laws which we would only be able to describe as ghosts, angels, gods, etc due to our feeble little monkey brains.
 
Do aliens exist? Very likely. Have they been here? I have no freaking clue and think it's unlikely based on logic but certainly not impossible. Are WE going to ever be able to visit "strange new worlds" with alien life like in Star Trek? Even if we managed not to completely destroy ourselves by the time we've created the technology it will not be for a VERY long time and I'm assuming that our species probably wouldn't even be recognizable as "human" anymore.
 
Freaky deaky.

Oh, they were here alright. Don't tell me you're all too young to remember "War of the Worlds"?
it was fought and won in one day. And because WWII began soon after, all memory of it has all but vanished...
if not for the one documentary film produced by the great historian Mr.Wells, there might not be any knowledge of it ever happening.
Some conspiracy theories theorize that WotW was the real reason WWII started.
Some idiot got insanely irate over the fact that the aliens chose to land in America instead of overseas.
Then they completely flipped when they found out we killed them off with our diseases.
Perhaps the first time history didn't repeat itself.
Usually it's the invading forces that bring in the diseases and kill off the locals.

I learnt everything I know from Python's Galaxy song.