the universe is definitely expanding at an accelerating rate. ofc that doesn't refer to some magic orb of existence growing outwards, it just means that space itself is kinda "stretching out."
dark matter is actually more like
27% of the universe's mass and energy. you may be getting confused with
dark energy, which is used to explain how the universe is expanding in our current models and makes up nearly 70% of the observable universe's energy (but it's spread throughout all of space so it's not very dense)
It was actually proven that whatever the forget dark matter or dark energy is doing it makes no loving sense and the laws of physics behind it make no sense.
dark matter and dark energy are actually two different things, and they're ideas that came about essentially because they
have to exist for our current models to work and accurately explain what we can observe. or at the very least it makes it a lot easier on us