Is there a definite number of atoms in the universe?

[Geekchick]

This may be a silly question but it just popped into my head yesterday. Are there a definite number of atoms in the universe? Because if the universe is expanding, then does that mean that there are more atoms in the universe or is everything just being pulled farther apart? (however, that logic really does not make sense because as things move apart then something else moves into taking up the newly empty space) If, however, there are more atoms then where could they possibly be coming from? Does this just lead back to how was the universe was created in the first place?

 

[mgb_phys]

The big bang model is that pretty much everything was created at the start.
It took a while for the universe to expand and cool enough that energy could convert into matter and then it took a bit longer for atoms to form from other particles.

So in general the amount of mass+energy is fixed. Of course atoms aren't indivisible so for example new helium atoms are being created from hydrogen atoms in the sun, which since it takes 2hydrogen atoms to make one helium means the total number of atoms in the sun is slightly decreasing. Other processes are splitting atoms apart.

As to the expansion of the universe. Although space is expanding, gravity (and other sub-atomic forces) holds things together. So on a very large scale things are moved apart, but gravity keeps anything smaller than galaxies together.