"There are known knowns, there are things we know that we know. There are known unknowns, that is to say, there are things that we now know we don’t know. But there are also unknown unknowns, there are things we do not know we don’t know".
Matter is stuff, that is a known known. Dark matter is stuff that we know is there, but we don't know what kind of stuff it is. It doesn't radiate or omit light. We can't see it, touch it, feel it, smell it or taste it.
"How do we know it's there then"?
"Because when scientists look at the universe they see that clusters of galaxies and gasses stay together, even though the stars and galaxies don't have enough mass, or gravity to hold them together. It has been calculated that we only see 20% of stuff, but there's 80% of stuff that we can't see, but we know it must be there. This is an a known unknown.
Newton's law of gravity and the "Big Bang" theory are known. If it turned out that they are wrong, then they would become unknown unknowns, because we don't know we don't know them.
I hope I've cleared that up for everyone".
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