Yes, matter can be something that you can not see. This is because one form of matter is gas, which can not be seen.
We use dark matter to explain the missing matter in the universe, for example you study the stars and you see they move but you can't see what gravitational pull is pulling it. We don't know what actually is it, how is it. This Means This Cannot Be Answered Yet.
No, because matter is something that takes up spaces, and matter is something that you cant see.
Sound cannot be Matter as we can not see sound and it has no mass
you can only see when its light and when its dark you cannot see
matter is something you can see or touch energy is not.
Yes, true steam (you cannot see it) is matter in the gas form.
The clear answer is, we don't. We cannot detect the hypothetical "dark matter", and the only reason we are talking about "dark matter" is that we cannot actually see enough mass in the Milky Way galaxy to account for the gravity that we know must be there - because the Milky Way would fly apart with only the mass that we can see. The "dark matter" may be in the form of invisibly-dim brown dwarf stars, or black holes, or "something else". Dark matter is the "something else". Everything you read about dark matter is a guess.
microrganisms
The air.
If the person is someone you know, the dream means that you do not clearly understand this person. Consider how the word "see" is used in conversation. You might explain something then ask, "see?" in the sense, "do you understand what I said?" The dream puts that into a literal image; you cannot see (the person) in the sense that you fail to understand.
The way I see it is that neither one is true, we don't (yet) know the true nature of matter, and that the best we can do is to use the models we do understand to explain it.
Well if you cannot see the model you are making you can use your hands by feeling what you are doing.