What I am given to understand is that Earth is re: as the center of the universe. It is not a planet as you would ordinarily define the word. Rather, Earth is something wholly unique. Everything circles round us. Excelsior's explanation is the best I've seen so far.
Nothing can exist in a vacuum, unless it's equipped to seal itself against the zero psi of it, which earth does excellently, because it sorts out it's elements from heaviest to lightest.
Earth like cells can exist in suspended animation which is what the vacuum would render all cells, but we could never see them as we can not see through something that does not exist. We only see it as darkness or blackness.
Think of it like this. Imagine you had a fish bowl on your head and your friend had one on his head. Now imagine that inside this fish bowl, you have everything that earth has and it's all lit up inside the fish bowl and you can look around it. BUT, someone covers your fish bowl with dense black cloth and also does the same to your friend.
You can still see around your fish bowl environment, but you cannot see your friend who is maybe a few feet or even inches away from you.
You exist in that fish bowl and everything you see is inside that fish bowl and like wise for your friend.
You know he/she is there, only because you saw each other before the experiment.
If that was you inside earth, you can guess there could be other cells but you will never ever know, yet it doesn't stop your conscious thought from telling you there is another earth, just as your conscious thought tells you (with help from TPTB) that you can see into space, because you can see the sun/moon/stars/galaxies, yet they are all inside the fish bowl. Can you see what I'm getting at?
What we see, or think we see, is not what's really happening. It's far too hard for many people to grasp, because their heads have been crammed with numbers and fictional calculations of star distances and what not, to the point that they cannot rationally think of something like this being what earth is.
Space does not exist. All that exists is us as earth and what is within it and MAYBE other earth like cells, not necessarily the same as ours, as we see it.