In space, our Universe models become muddled.
It all begins with dark matter, or lack thereof. Dark matter causes spiral galaxies to spin faster than they should, but it doesn't address all our concerns.
Modified Newtonian dynamics (MOND) is a dark matter alternative.
Mordehai Milgrom's 1983 hypothesis says we don't need dark matter to fill the Universe's gravity gaps if we calculate stars' gravitational forces differently than Newtonian equations predict.
To test this concept, which includes proportionality to the star's radius or centripetal acceleration, we must look at galaxies' speeds, notably ultra-diffuse galaxies.
These weak, ugly duckling galaxies function strangely. Some very diffuse galaxies appear to be virtually totally dark matter, whereas others are dark matter-free.