Astrophysics professor Matthew R. Buckley, one of the last people at Rutgers University who still thinks, decided to prove something he thought was true: Dark matter, the mysterious substance theorized to be the bulk of the universe's mass even though it can't be seen and has yet to be detected, couldn't form massive structures like planets.
Buckley figured that dark matter would not be able to release heat like normal or "baryonic" matter. It doesn't have photons like the everyday stuff does (one of the reasons it's dark and not detectable by any method that uses light), and photons play an important part in allowing clumps of atoms to relase the heat that they generate as they gather. If they don't release the heat, they can't solidify.
But if dark matter had a process to shed the heat, equivalent to electromagnetism in baryonic matter, for example, then it could clump together into large structures. It might even be able to form dark matter galaxies. They wouldn't last long, though, because among the other things baryonic matter has are forces that prevent gravity from collapsing its large clumps into black holes unless they are under extreme conditions like a supernova.
Dark matter planets, on the other hand, would be small enough that gravity wouldn't exert as much pressure on them and they would not collapse like a larger structure would.
Since dark matter emits no radiation, it would be almost impossible to find such a planet except by accident, such as being close enough to it to be affected by its gravity. And as Buckley points out, landing on one would be difficult. Despite our solid appearance, we are mostly empty space at an atomic scale (Insert Donald Trump or Bernie Sanders joke here according to your own personal preference). So is the Earth. Something called electrostatic repulsion keeps our atoms from sinking into the space between the Earth's atoms. But there would be no electrostatic repulsion between dark matter and baryonic matter, so we would just sink through to the center of the planet as its gravity drew us inward.
The speculation about a dark matter planet is pretty interesting, but one of the neat things to me was the way that Buckley and his colleague Anthony DiFranzo set out to prove one thing and wound up offering an entirely different idea when the evidence led them that way. Now that's science.