Our current mathematical models are pretty damn close to how stuff works in reality. But like you said, for it to work out, you need to fudge the numbers a little. That's dark energy.
Dark matter is, as far as we can tell, a real thing, just really hard to detect. It's not a placeholder in the same way dark energy is. Something out there is exerting actual gravity.
Millions of man-hours have been spent on this. No one is going to prove or disprove it for you in some Lemmy comments. I would seek out talks from actual astrophysicists, not YouTube junk pop science.