My understanding of the indicator of dark matter is the rotational speed of galaxies, specifically how stars further out rotate at higher speeds than what is expected from the gravity of normal matter - there is something exerting a force pulling it in, that acts like extra gravity.
Then, dark energy is a force that causes space at large scales to expand.
If there's some force stretching space at large scales, then surely there would be a resultant counter force that pushes at smaller scales (of galaxies)?
Probably a bad analogy, but something like a super slow time lapse of what happens after letting a firework off in a glass jar filled with tiny bubbles - after the explosion, there is a rapid outward expansion (assuming the jar has exploded) and everything is expanding outwards, but the high pressure that created the explosion created localized pressure. The bubbles in the jar (galaxies) are still experiencing pressure / compression as the expansion goes beyond the original boundary of the jar. From an instant in time just after the explosion / jar shattering (in super slow time lapse mode), it would seem like the bubbles are heavier than what they are / spin quicker (dark matter) because of the force of the explosion on them (dark energy).
So maybe there isn't "undetectable stuff that adds gravity", it's just the pushback of a rapidly expanding universe
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