It was proved by Cantor in the 19th century, and writing to another famous mathematician for comments, he famously included the line "I see it, but I don't believe it" (actually, the line was Je le vois, mais je ne le crois pas). The most famous proof is one I used in my thread on infinities and is called a diagonal proof (other mathematicians used this method of proof, including Turing when he basically founded computer science and "invented" computers, but I think Cantor was the first). This isn't, though, the only way to prove that result or similar results, such as the fact that the power set of the natural numbers is as large as the set of real numbers and as large as the set of numbers in the interval (0,1).Assuming you are right, and there is no reason not to, this would make a helluva bar bet.
That said, there is a difference between the fact that the set of real numbers is larger than infinite sets like the rational numbers or integers and the "fact" that there are infinitely many infinities. If it were false that there are more real numbers than rational numbers, most of mathematics would come crashing down along with all the technology that required it. Important numbers like pi couldn't be used in most applications, high school students wouldn't be able to learn any algebra that involved graphs of functions, most of physics would fall apart, etc.
This is not true of the assertion that there are infinitely many infinities (understood to mean we can order them such that each one is larger than the one before it).
The simplest way to prove that there are infinitely many infinities is to use the power sets. This makes sense when we are thinking in terms of finite sets like the set {1,2,3,4}. Given that set of 4 elements, or any other set, the power set contains all subsets and the set itself (technically, every set has itself as a subset). I won't give the proof, but I think it's fairly intuitive that given a set (a collection of elements/objects), something that includes all of those elements as well as collections of some of the elements (subsets) has to be bigger, because it includes the whole set and more.
The power set of a set is defined this way, such that it includes the set and all possible subsets as well, so it makes sense that it would be bigger.
...Until we get to infinite sets. It is not as easy to provide the kind of incontrovertible (if wholly counter-intuitive) proof that the real numbers are a larger infinity than e.g., the rationals here. In fact it is proved by using the clearly true proof that for finite sets, the power set is always greater than the set, and saying this holds true of infinite sets. There are in general two kinds of proofs. One kind consists of proofs of existence. I can prove to you, for example, that if I add up the terms
The proof that shows this is true for all such infinite summation is an existence proof because, given some value of p greater than 1, it only tells you that the summation of infinite terms will converge; it doesn't tell you what it will converge too,