system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we need to postulate the presence of dark matter? Wouldn't unexpected astrophysical phenomena simply indicate that our understanding of gravity in inadequate?

It seems to me to be a great jump to postulate the existence of a new type of matter in response to astrophysical observations not lining up with predictions as dictated by current theory. Would not the more parsimonious response be to work on modifying the theory? Do we really need dark matter to explain our observations?
gpt
People seem inordinately bothered by dark matter.  Dark matter is evidence that our understanding of gravity is good, not the other way around.

Let me begin with another situation.  You hear your doorbell ring, yet when you go to look, there's no one there.  Do you suppose that there must be something new electrical force that only shows up in doorbells or do you figure somebody rang the bell and then left?

Or imagine you see a star's light showing oscillations in its wavelengths, up a little bit then down a little bit, in a regular pattern.  Do you imagine that these particular stars are governed by some new force observed nowhere else in the universe, or do you figure that you've detected the influence of a planet -- which would produce exactly this effect as it orbited the star -- even though you don't have an actual image of the planet?  That's exactly how the first exoplanets were discovered.

Or imagine you see a loss of energy and momentum in a decay process.  Do you imagine energy and momentum conservation should be tossed aside, or do you recognize that there is a particle you've not overtly detected carrying that energy and momentum?  That's exactly how the neutrino was discovered.

Or imagine you see the stars near the center of our galaxy undergoing elliptical motion around nothing that you can see.  Do you suppose there is something wrong in our understanding of motion, or do you conclude that there is a massive object you can't see at the center of our galaxy, around which things are orbiting?  You conclude the latter, and have evidence for the black hole at center of the Milky Way.

Dark matter is evidence that our understanding of gravity is really, really good.  Remember, general relativity has survived a range of tests, from the orbital mechanics of the solar system and tides on Earth all the way up to cosmological behavior of the universe and the merger of black holes.  Positing dark matter *is* the parsimonious approach, because it turns out that we can explain a host of phenomena--the motion of galaxies, the motion of stars within galaxies, gravitational lensing around galaxies, and features of the cosmic microwave background radiation--if there is additional matter that interacts gravitationally but not otherwise (or perhaps also via the weak force).

If our models of gravity were wrong, positing dark matter would not be expected give us results consistent with all these quite varied observations.  

I'll add that we know that not every particle feels every force -- electrons don't feel the strong force, for example--but every kind of object is subject to gravity.  Therefore, there is nothing particularly peculiar about something that does not interact via any force other than gravity.  (And, since dark matter may interact via the weak force, that would put it in the same category as the neutrinos, which feel the weak force and gravity, but not the strong or electromagnetic force.)