The Kardashev Scale
Another way of talking about this is to say that if you were to get a giant excavator to pile together every single bit of sand found on our entire planet, you would have to take each grain and multiply it by 10,000 to arrive at the number of stars in the universe. Next, factor in the age of said universe (13.8 billion years) and the relative youth of our planet (4.5 billion years), and it begins to seem extremely improbable that more than a few advanced civilizations haven't popped up here and there over the eons [source: Foley].
That said, as mentioned earlier, to achieve interstellar travel is no walk in the park. For starters, it would require access to vast quantities of power.
A Russian astronomer named Nicolai Kardashev came up with a handy rubric for different types of likely civilizations, catalogued in terms of power usage: types 1, 2 and 3. We're a century or two away from becoming a Type 1, which is a civilization that has advanced enough to be able make use of all of the available power on its planet.
A Type 2 civilization would be able to tap into the power output from its local star. Imagine if we could get an extension cord to the sun! All of our energy needs would solved. Naturally, you can't actually plug into the sun, but maybe we could use something like the Dyson sphere, a theoretical technology that wraps an energy-capture system around the sun and absorbs all of its output.
The astronomy world actually has been thrown into a tizzy by a sun known as Tabby's Star, located in the Cygnus constellation roughly 1,480 light years from here. It appears to have been dimming progressively and quite mysteriously over the years in a unique way. One theory is that an alien civilization is in the process of building a giant Dyson sphere around the star, slowly cutting it off from view [source: Swan]. If Dyson spheres really are popular with Type 2 civilizations, this might explain why we can't hear them; their radio signals never make it past the megastructures they've constructed around themselves.
Moving on, a Type 3 civilization on Kardashev's scale would be able to harness the energy output of an entire galaxy [source: Foley]. A species that has reached that level of sophistication and sheer power would have about as much time for us puny earthlings as we have for dung beetles. Their forms of communication might be completely unrecognizable to us, which would help explain why we can't detect their presence.