Santa is one of the survivors from the very advanced civilization that existed on Atlantis and uses technology not yet available to us to deliver his presents.
Putting it simply for the non-scientific this is the story…
Santa generated a wormhole, where one area of space short-circuits to another: a tube with a mouth at each end, which acts as a kind of short-cut to get from one place to another without going through the intervening physical space. He then took one mouth of the wormhole on a near light-speed trip around the solar system to benefit from time-dilation (Lorenz-Fitzgerald effect). Bringing the wormhole mouth back to Earth and his hidden laboratory deep in the bowels of the planet (at the North Pole of course - in fact, the Earth's magnetic field is generated not by natural causes but by the field generator Santa uses to keep his wormhole open) he now has one wormhole mouth that is "younger" - the time-dilated one effectively exists back in time compared to its sibling that stayed on Earth. So when Santa steps into the "old" wormhole mouth, and moves the small distance in space across his laboratory to step out of the "younger" wormhole mouth, he moves not only in space but in time. So, when he steps into one wormhole mouth and comes out the other, he comes out before he stepped in. This means there are then two of him. So both of him can step through the wormhole and then there are four Santas… and so on.
Thus, Santa can multiply himself exponentially at Xmas time in order to deliver all the good kids presents practically simultaneously.
Of course, they all have to remember to step back into the wormhole at the right time, or they wouldn't have existed in the first place, so whatever you do, don't detain Santa - he is still in a hurry even if he can be everywhere at once.
It also explains how he can drink all that sherry and eat all those mince pies.