What a lot of questions! As a starter to a starter, I would like to comment.
Why do people think the universe is finite? - It depends, whether you mean *some* people or people in general. Some people think the universe must be finite because they consider infinity as a construct of the human mind that does not exist in the "real" physical world. In general, there is no consensus about this subject. However, we do agree that we can see only a limited (finite) part of the universe because of the finity of its age and of the speed of light.
Why do they assume that the big bang is the origin of the universe? - We know for sure that the universe must have been very hot and dense some 13,7 billion years ago. We can derive that from the cosmic background radiation and from the ratio of isotopes of Hydrogen, Helium and Lithium. Whether you want to call this "the origin" is a matter of taste. You may postulate deeper causes of this dense state, but you cannot go on forever defining deeper causes, since we ourselves are finite.
While the big bang might have occurred, it is not the "origin" of the universe. - Same answer. You may try to question why the universe consists of certain particles, why the fundamental constants have a certain value, or even why there is a universe at all. Some progress is made in this field, like the inflation theory, but an answer could never be final because of reasons given.
It at most is the origin of the expansion of matter through the universe. - Sure, but why is it that the universe has to expand? Yet another question.
Think about the universe as a huge infinite vacuum that contains matter within it. - Thiws is a dangerous concept. We used to think of space and time as a sort of stage, on which all matter, energy and events take their place. However, it has become clear that space and time - and so vacuum as well - are real players in the universe; not merely a decorum.
The vacuum is infinite. Why? For the simple reason that it is not matter. It is nothing actually. It is the absence of matter, of existence, of time. - No, the vacuum is an intrinsic part of the universe. Like the universe, it could be finite or infinite. We don't know.
That kind of thing does not need to have "edges" or "origins". - I do not agree. Something can be finite and yet have no edges: like the surface of a sphere.
What is finite and what has edges is matter but again, i think scientists are confused as to what is the universe, and what is the matter within it. - There's no confusion about that. You need space to have matter and you need matter to have space. You could do thought experiments on a conceptual empty universe that does not contain matter or anything, but you could never prove or disprove that such a universe exists.
The universe is not a box or something that is composed of anything other than pure nothing. - You could argue that way. One concept of space-time and matter-energy states that the total amount of energy in the universe is zero, and that all that we experience emerges from a quantum fluctuation. But in practical life, this "nothing" becomes very real!
Or maybe I am getting something wrong, what do you think? - I think it is nice to speculate about the deeper causes of our universe and our own existence. But science must stick to the facts that we can observe, however limited these may be.
