Dark Matter is not just 'something we don't understand yet'.
It has predictable effects on observations and adheres to some properties, see: https://en.wikipedia.org/wiki/Bullet_Cluster .
Re:A Physicist Who Denies Dark Matter
by tynpeddler on Sunday May 21, 2017 @04:40AM
(#14386357)
That's evidence, not proof. We still don't have enough evidence to say conclusively that Dark matter is as we currently imagine it. And of course the same can be said about MOND.
Here's Milgrom's interpretation of the bullet cluster: http://www.astro.umd.edu/~ssm/mond/moti_bullet.html
I am not saying Dark Matter is something that actually exists that cannot be ruled out by other explanations (a revision of GR would be ideal IMO), just that this phenomenon makes the approach much trickier.
I can deny the Earth is flat, because there is an abundance of evidence that it is round.
I can't deny that there are 650 elements, because as far as I know, other elements may be discovered. But I can certainly doubt it, and supply alternative theories.
I can deny allegations of wrong doing, because they either happened, or didn't.
But Dark Matter isn't assured yet. Denying it means it either exists or doesn't. But the truth may be a mix of MOND and something different but similar to Dark Matter. It isn't "fixed" into a yes/no situation.