Where are the aliens?
I'll elaborate on that question if you'd like.
To quote the wise Calvin:
"Sometimes I think the most sure sign that intelligent life exists elsewhere in the universe is the fact that it hasn't tried to contact us yet."
Please do.
Because of the vast size of the galaxy it's entirely possible for life to be common in terms of absolute numbers, but yet to be so thinly dispersed in space that we are too far away from the nearest neighbor to readily contact or detect it.
Also possible that life as such is common, but the evolution of intelligence (and technological civilization) is rare.
Even if intelligence and civilization is common, it's possible that interstellar travel is a practical impossibility, or too costly to be a desirable activity. Communication by radio or other electromagnetic signal might also be too expensive (if you want to make the signal detectable over really long distances). Even if this is not so it's possible that civilizations might decide, once they have the ability to broadcast strong signals ubiquitously, that it might not be such a good idea to betray their location to unknown others.