Re: how can we presume to know anything about the 'universe'?

You're right that this is a philosophical question - part of the branch of philosophy called "epistemology," which tries to understand how we can really "know" anything - but it is also something that scientists have to address. It is certainly true that we can only perform experiments on a very small piece of the universe. Presuming that we do truly understand physics on earth, for example, how can we assume that the same physical laws apply elsewhere?

The answer, as you are arguing, is that we can't know with 100% certainty that the laws of earth apply everywhere else. It's an assumption, and the question is really whether it's a reasonable assumption. I - and I think every other working astronomer - would argue that it is probably an excellent assumption. For me and for most people, the reason comes down to simplicity: a single set of laws is the simplest possibility, and scientists tend to like simplicity. It's elegant and, so long as it seems to work, there's really no point in making things more complicated than they have to be.

An important premise of this assumption is that, so far as we can tell, our laws do work extremely well. We can explain phenomena that we see in the distant universe quite accurately using our simple laws. We also detect light from the distant universe, and that light seems to obey the same rules as light from nearby. Until we encounter something which we truly can't explain, we might as well make the simplest assumption possible and use the same set of laws everywhere.

Another question is where to draw the line of "close" (where our laws would apply) and "distant" (where they wouldn't). For example, Newton developed his laws of motion in England. But obviously they work just as well in the US, China, on the moon, etc. They also work fine throughout our galaxy. Again, if we can't find a region where they don't work, then we might as well assume that they do.

Nevertheless, there are possible ways in which the distant universe may differ from our own. One example that astronomers have long considered is whether it's possible for other galaxies to be made of antimatter rather than "normal" matter. It turns out that antimatter can form atoms and molecules and planets and stars and whatever else...if a galaxy made of such antimatter objects were far from us, we'd have a hard time telling the difference because, in most respects, it would look like a normal galaxy. The only observable implication is that, at the edge of this island of antimatter, it would eventually encounter some regular matter. When matter and antimatter meet, they annihilate and create characteristic forms of radiation. We'd thus expect to see this sort of radiation surrounding the antimatter-dominated parts of the universe. We haven't so far. That doesn't mean there aren't such antimatter galaxies - perhaps the radiation is just too weak for our telescopes to have seen it. But astronomers will assume that they don't exist until such radiation is seen - because that is the simplest thing we can have.

There are many other examples of assumptions we must make about knowledge, many of which were pointed out by David Hume, a Scottish philospher in the 1700s. The most famous is "induction." How do we know that the sun will come up tomorrow? We don't - we just assume that, because it has every other day of our lives, it most likely will tomorrow. But that's just an assumption! Also causation - suppose one billiard ball hits another, and the second one rolls away. Most of us believe that the impact "caused" the second ball to move. But do we really know that? Maybe it was just a coincidence. The point is that these sorts of assumptions are necessary in order to make any progress in understanding the universe - and even in leading our normal lives - so I do'nt find them unreasonable.

In summary, you are correct that this is an assumption that we must make. But it is a simple one and, so far as we can tell, it is correct. It may be proven wrong sometime in the future, but until that proof is in place expect scientists to continue to believe it!