Mimi wrote:

But the universe is infinite.

The universe may or may not be infinite - we cannot know. But the

observable universe is definitely finite. Light travels at a fixed speed [1] and the Big Bang was a fixed amount of time ago. Therefore, there is a maximum distance light can have travelled between the Big Bang and now, and we cannot get any information about anything further away than that. So we can talk about "all the hydrogen in the observable universe" meaningfully.

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So there most surely be an infinite amount of hydrogen. And infinite amounts of the other elements, so how is one more abundant than the others?

As others have said, some infinities are larger than others. Consider the set of integers (often written as Z): 1, 2, 3, 4, 5, ... There's an infinity of them. Now consider the set of real numbers (often written as R): 1.1, 1.2, 1.25, 1.257, 1.2573, 1.3, 2.0, 2.4... There's an infinity of them, too. But every integer is also a real number, and so R contains Z, and has extra numbers on top. Therefore, the size of R is greater than the size of Z, even though both are infinite.

[1] in a vacuum, anyway