✔ 最佳答案
within a few orders of magnitude, depending on how you want to define the word "universe". the number is pretty large. We can come up with a reasonable estimate of the mass, and that pretty well imposes the number of atoms, given that hydrogen is by far the most common element. The mass estimate depends on a lot of fairly uncertain parameters, of course (average mass of a galaxy, number of galaxies, those sorts of numbers), and obviously we cannot define what we cannot see, and we cannot see the entire universe. It is not possible to give a limit to something that has no evident end. And it doesn't matter at all whether you assume hydrogen is all mass, or only half the mass (or something else). the uncertainty elsewhere is far larger than a factor of 2.
It is just a standard back of the envelope calculation (rough estimates for all important factors will give you a rough estimate for the total).