Theories are descriptions, and they come in two flavors: true and false. So the Theory of Evolution can be both a theory and true, which is just what a great number of scientists believe. When evolution by natural selection is called a theory, however, this is sometimes intended to emphasise that there is no proof that it is true. Now if by 'proof' we mean what pure mathematicians produce, then this is correct. There is no proof of the Theory of Evolution, and there is no proof of any other empirical theory either. Proof in this sense is not an option in science, because all theories go beyond the evidence upon which they are based. There can similarly be no proof that the sun will rise tomorrow. But the sense in which it is true that there is no proof of evolution is compatible with the claim that there is overwhelming evidence that it is true, which is again what a great number of scientists believe.
I'm going to say something here that is way over-simplified, but perhaps it will do.
According to quantum mechanics, of which my knowledge is very limited, such things as atoms don't have distinct boundaries in the sense you have in mind. This is because their parts (protons, neutrons, etc) don't have completely determinate positions in space (except under certain exceptional assumptions). Rather, the location of a given particle is described in terms of the probability that it is in a particular location. In fact, the same is true of macroscopic objects. Even waiving the blurriness of my boundaries, my location is not completely determinate, either.
It therefore seems reasonable to say that, no, it isn't completely determinate where your finger ends and the key begins, even if we can say which particles constitute the one and which the other (another hard problem), because it isn't determinate where those particles are.
People who believe that science is a social construct do not deny that there is a world independent of ourselves, with real causes and real effects. What they emphasise is something consistent with this, namely that the theories that scientists invent to account for the world are strongly influenced by social context.
The following comment has been kindly sent in by Professor Kannan Jagannathan (Department of Physics, Amherst College):
"The best evidence we have for the isotropy andhomogeneity of space leads cosmologists to hold that the universe hasno center and no periphery. If the universe is infinite now, it was soat the Big Bang, and the bang occurred everywhere (in such a case, thedensity of the universe would have been infinite at BB); if theuniverse is finite (and unbounded) now, it was probably point-like atBB, but it is not to be thought of as embedded in some larger space. That was all there was as far as space was concerned; it was'everywhere' then, and is everywhere now.
In the standard model of cosmology, as well as inmost variants of it, the initial rate of increase of the scaleparameter (crudely, the radius of the universe, or the rate ofexpansion of space) would have been bigger, perhaps much bigger, thanthe speed of light. The combination of these two points would suggestthat if light was emitted at BB, we would still be receiving bits ofthat light from parts of the universe that had sped away too fast, andwe would continue to do so for the foreseeable future, particularly ifany of the inflationary scenarios is taken seriously.
The reason the earliest light is from something like300,000 years after the BB is because that is when the universe hadcooled enough to allow neutral atoms to form, and the universe suddenlybecame transparent to electromagnetic radiation. Prior to thisso-called 'recombination era' (a misnomer since it was the first timethat the 'combination' could have occurred), the universe was opaque toall 'signal carriers' that we can think of or detect easily; if onegoes a little farther back in time than 300,000 years, even theneutrinos would have scattered too much to be available as a faithfulimprint of events before."
I'm not a philosopher of science, so I have no first-hand knowledge here. But a search of Philosopher's Index turned up a review by Jeffrey Koperski, in the British Journal for the Philosophy of Science 2001, of Peter Smith's Explaining Chaos. I also found a few papers on the relation between chaos theory and quantum mechanics, in which there is, apparently, no room for chaos. See, for example, Gordon Belot's "Chaos and Fundamentalism", Philosophy of Science 67 (2000), 454-465.
Oddly enough, most of the references I discovered were to papers in philosophy of religion....
I think most scientists would reject an assumption in your question — that they believe the Universe needs to be one way or the other. Theories in science do not say how matters must be, they describe how they are.As to why scientists think the Universe does have a beginning, well,presumably it's because that hypothesis best fits the availableastronomical data. If you want the details, talk to yourlocal physicist.