The one thing Scifi keeps ignoring is just how huge and empty space is ...
Yes, there's well over 100 billion stars in this galaxy, and we know of at least 100 billion galaxies, so ... there is a chance there's life somewhere.
However the Sun is an exception: Most stars are binary systems, and stable orbits are unlikely near those. Also the Sun is in a safe location, since it follows a galactic arm at some distance. If we were in that arm where new suns are born and old suns explode every couple of 100k years, or close to the galactic center, we'd be more likely to be hit by supernova explosions, or have something heavy alter the orbits of the Sun's planets. Stars in a stable, safe location like ours are quite rare. (Also life can probably only exist near Sun-sized stars: Small stars like brown giants are too cold, big ones send too much dangerous radiation and don't live very long before exploding.)
Also, to make life, one needs heavy elements. These did not exist in the early universe - after the big bang, there was only energy; when that cooled down, it became mostly matter and antimatter which made more energy; some particles remained -- Hydrogen and Helium.
In order for us (mostly carbon, with some oxygen and iron etc) to come around, the Hydrogen and Helium had to form stars, which then fused Hydrogen, ran out of hydrogen, burnt at hotter temperatures creating heavier elements up to iron, and exploded in supernovae creating elements heavier than iron, and forming new dust clouds.
This must have taken a while - supernovae need to build up and go boom (tens of million years only, for very heavy stars), the escaping material needs to form dust clouds, these need to form into new stars ... eventually you'll get dust that is rich enough in heavy stuff to make earth-like planets.
Scientists estimate this would take some 10 billion years.
Earth is like 3 billion years old, and the universe 13.8 billion years if I remember right -- that means the Sun is probably among the first suns to even have enough heavy matter to form an Earth.
Still, with the universe as huge as it is, it must have life somewhere.
The size is another problem: No information can go faster than the speed of Light ... and that has been very much proven in Physics. Travel to somewhere that's even a hundred thousand light years away is pretty much infeasible if you consider the lifespan of human civilizations ... the whole planet would need to cooperate just to send a handful of people away, to never hear of them again. Likely?
Sorry I sound like such a pessimist. Source for my badly documented rant above is in an excellent astronomy lecture series by an astronomy prof, in German, which covers, among other things, questions such as "how real is Star Trek?", "are there aliens?" etc.
http://www.br-online.de/br-alpha/alpha-centauri/alpha-centauri-harald-lesch-videothek-ID1207836664586.xml
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Topic: Alien Civilizations - Galaxy has 'billions of Earths' (Read 20696 times)