First off, it's *possible* Earth contains the only 'advanced' life in the universe. (I don't think this is true, but.. it's possible.)
It may be that intelligence is extremely rare - and planets with advanced creatures are *very* far apart. If that's the case, it may be that Earth has simply not heard from - or broadcast to - any planets with intelligence as yet because our signals simply have reached that far.
It may be that other planets know about Earth - and, have answered - but, we're not smart enough to know how to listen for them.
It may be that other planets know about Earth - but their return signals back to us haven't arrived yet.
It may be that they're fully aware of Earth - and Man - but have chosen not to answer... They're either allowing us to mature, studying us as a society, or prepping for an invasion... Either of these choices is kinda spooky.
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Quadrillian say: Most likely because it is either non existent, or is so rare that there is none within detection range.
I don't understand the psychology of some people who can come up with the most bizarre explanations when the simple one that I described here will suffice to cover all known observations.
Cheers!
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Gary B say: because it is SAMLLER than a planet, and thus we can't SEE it.
Of the 3000 or so planets we THINK are there, we have only SEEN a few dozen of them.
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Johnny say: Because the distances between worlds is far too great for us to travel within our lifetimes. If a ship would leave today, it would be the great-great-grandchildren of the original crew who would be the ones who get there.
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Nyx say: Because distances between galaxies is huge, as in millions of light years apart.
And it could just be that for us, the local stellar population surrounding our star system, simply may not have any life on their planets. Or life that is incapable of achieving the equivalent level of technology that we currently enjoy.
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