Space has 3 large dimensions. If you spread out and mark different points in space, and measure the angles between them you can verify if you are using straight lines in the dimensions you think you are. Just like a 2D person could walk around on their paper and draw large triangles and measure the angles. If the 2D world is perfectly flat the angles will work out like they do in geometry class, but if it's warped in some 3rd dimension you will see discrepancies in the angles. If you start at equator, walk to north pole, turn 90* right, walk back to equator, turn 90* right, then walk to starting point you will just have made a triangle with 3 90* angles, which does not compute for a triangle, because you are trying to measure something in 2 dimensions when it is in 3 dimensions. When we measure space though there is no warping, just 3 dimensions. We may have some dimensions curled up real small, but I don't think there is any possibility there is another large one we aren't aware of.
An intelligent silicon based life form probably wouldn't exist. It makes many similar bonds as carbon, but it is a different size and does not quite have the versatility of carbon. I don't think silicon based life could be as complex, I think it would have to be carbon just like us. That's just how chemistry works.
