Most of the observations behind that fallacious pseudo-theory are based on observations of other galaxies.
Not arguing that the observations aren't real. Just every so-called "hypothesis" for DM is fit to a selected set of observations, and falls over once you bring in other data. In addition, none have yielded testable predictions that have succeeded - those that have made predictions, that can be tested, fail.
And let's not forget that the quantity of "missing matter" is continually being downgraded. To the point where we can probably explain the universal expansion without dark matter, but not the galactic rotation issues.
My personal theory is that if there is dark matter, it's the mass of grant application paperwork for academics who want to be funded to research dark matter. :flame:
Now Dark Energy - at this point, no-one's staking much ego on predictions. They're still observing, measuring, and validating. The theories are clearly initial best guesses, and no-one's creating an industry. But it does indeed look like there's some strange **** happening with gravitation on mega-scale distances that we don't yet understand.
re: DE
In GR, understand that you basically have
G = T
Curvature tensor = Stress-Energy tensor
On the LHS, you have something reflecting the curvature of the space-time fabric. On the RHS, you have something reflecting the mass-energy distribution within space-time causing it to curve.
If you add in a Cosmological Constant L, you get either:
G + L = T
-or-
G = T - L
If you move L to the RHS (G = T - L), you can interpret it as a stress-energy arising from an exotic mass-energy distribution having fantastical properties
If you keep it on the LHS (G + L = T), it merely expresses a sort of inherent curvature, present within the fabric of space-time, even in the absence of any mass-energy. So understood, it would simply say that, on the largest of cosmological scales, the fabric of space-time doesn't "lie flat", but has an "inherent warp", ever so vaguely like the upward bowing built into (some) flatbed semi trailers, such that they arch upwards in the middle when empty, and only lie flat under load
I guess you get better hype & headlines with the more exotic interpretation, which is more popularly publicized, even if the more mundane interpretation is equally valid