I believe the universe to be infinite because I must. I cannot prove or disprove infinity - I can't even fathom it. However, I am very familiar with finity, so I'll work with that in a moment.
Infinity seems intuitive enough, but I want to be precise with my terms so that I am understood and so the replies will be perfectly relevant.
Infinity is substance without boundaries.
If anything exists anywhere, it is necessarily a part of the infinity, because to isolate it would imply a boundary, and infinity is a "state" that involves no boundaries.
Finity, as the only alternative to infinity, implies at least one boundary.
Absurdity is a state of being that can not exist in reality because it is a contradiction by definition.
An example is a square circle - because a circle must have no corners and a square must have four, square circles are absurd, and therefore cannot exist, or cannot exist and are therefore absurd.
Substance is anything that exists. Anything at all.
Very quickly, I want to answer to the "First Cause" argument, which, forgive my stupidity, I cannot take seriously.
If we are to postulate a first cause for the universe, then three thoughts immediately come to mind.
The first thought is that through accepting the First Cause argument, one denies the infinity of the universe by default. I do not know if this is commonly accepted as true or not, but it most certainly is true, because creation implies beginning, which in turn implies a boundary.
The second thought is that the function and primary assumption of First Cause is that nothing may exist without a cause. This appears myopic to me- everything within a specific and measurable timeframe is subject to cause an effect in a universe that exists in time and therefore by definition can not sit still even for a moment (which would contradict the existence of time, as a measurement of motion). But there is no reason that the whole must be subject to cause and effect, and in fact, it cannot be, because of the infinite regression we reach when we ask "what caused the first cause?"
Infinite regression implies a dilemma.
Either - there is absurdity through a statement that defeats itself: All things must have a cause, but the First Cause doesn't have a cause, or there is infinity.
What is a boundary? It separates one substance from another - that's it.
Nothing - by definition, does not exist.
Let me present another dilemma that is similar to the first. If an object, such as the universe, is finite, it has a boundary. Therefore, the boundary either separates it from something, or it separates it from nothing.
If the boundary separates it from something, then we branch off into the original dilemma of absurdity vs. infinity.
If the boundary separates it from nothing, then we have simply described infinity in a roundabout way. To say "I am separate from nothing" is to say "I am infinite."
I'm getting a little tired right now but let me just wrap this up by saying the most common logical error made in this issue is assuming that cause and effect must apply to the whole.