There's a bit of a contradiction in what you say here.
Some sort of effect has been observed and measured, which is why the idea of dark matter sprang up in the first place. The predictions of dark matter "didn't fit," i.e., the observations and measurements weren't quite right ... so they added another term.
Blaming the discrepancies on some sort of matter and energy is a pretty obvious choice for explaining phenomena related to the interaction between matter and energy....
The fact is that something is causing those observed and measured phenomena. Another fact is that we cannot see what's causing it. Either there's something out there that we cannot see, or there's a problem with the theories ... or both.
Observing and measuring a phenomena is not the same as observing and measuring a substance like matter. They observed a phenomena they couldn’t explain so they fabricated something they can’t prove as a way to explain it.