I think finding a world with the same composition as ours is moderately unlikely, but we have found similar ones! There's just so much out there that it seems silly to assume that planets like ours are uncommon. An interesting (and rather egotistic) part of the Rare Earth hypothesis is that a planet like ours is the only place intelligent life can come from - there has to be other life out there. Primitive precursors have been created in labs with nothing more than minerals and energy (and I don't have a link to that, but if I find it I'll add it in), two things very abundant out in the big wide world. Things have a habit of surviving and adapting despite insane conditions, as volcanoes and sea-vents show. Intelligent life will show up somewhere, eventually, though it may not match our definitions (or even be carbon-based).
[x] Bob-IT [x]
For all we know, there could be no complex organisms outside of Earth. Perhaps we're the first to evolve and there is no complex life besides us out there yet.
There is just so much we don't know yet. But I'm SUPER excited to find out.
Have you read any David Brin (the writer and astrophysicist, not the other one)? That's one of the possibilities he has brought up - there's hundreds of races, they're going to bicker and trade and travel- and we're too early. No one's evolved within shouting distance yet. "The Crystal Spheres" (a fantastic science fiction story) is a logical extreme of the idea (which he referred to as "too improbable for my SETI presentations, but worth writing down'').
Yeah, we understand an estimated 4% of the universe. There's
so much more out there, and it's going to be
fascinating!