science

There's a few other things that dark matter explains which MOND doesn't, such as the relative abundances of the light elements and galaxies with seemingly no dark matter present.

One of the things with dark matter (specifically WIMPs, Weakly Interacting Massive Particles) is that they don't seem to interact with any force other than gravity. This means they can shed momentum, by interacting with each other, or normal matter. In order for a dense object to form from a cloud, the cloud, as a whole, must shed, or cancel out the momentum of the particles involved. WIMPs would not be able to do this via anything other than gravity. This limits their collapse to around galaxy scale.

In short WIMPs can't form anything more complex than an incredibly diffuse inert gas cloud. We can only detect those clouds on a galactic scale, via their gravity effect.

One of the recent theories I've heard is that dark matter is made of particles with a wavelength measured in light-years. They can't be localized to something as small as a star system, much less a planet.

Are there planets formed from Dark Matter?

Last time I looked, planets didn't glow.

They are just not numerous enough to explain all the things dark matter is supposed to explain.