Okay others have answered in more detail but this is more a short “Dark matter for Dummies” - explaining what it is if you have not much physics background. Dark matter means matter that we can’t detect easily. Doesn’t emit light, or reflect or interact with light in any way so we can’t see it. Dark matter also has to have mass, not just energy. Photons have energy but no mass - they travel at the speed of light and strictly speaking they have “zero rest mass”. They have a mass but only the mass of the energy that makes them up, not mass like ordinary matter.

We know that there are some very elusive hard to spot particles as the neutrinos fit that bill nicely. They are so hard to spot that for a long time they were just a hypothetical particle invented to explain certain experimental results. Nobody had “seen one”. This was the first “observation”:

Here, a neutrino has come into a bubble chamber from the right, but it doesn’t leave any trail. It hit a proton and then that lead to three trails from the movement of the proton, of a mu meson, and a pi meson. They curve in different ways in a magnetic field. They have no charge. They travel at nearly the speed of light but not quite. They interact with matter so weakly that only half of the neutrinos would get absorbed in a sheet of lead a light year thick.

If neutrinos have rest mass and not just energy, then they are “dark matter”.

So the idea of dark matter is that neutrinos may have mass, and that there may be many more particles as elusive as the neutrino and even more so.

They are usually thought to be "weakly interacting" so that they can't form molecules for instance. Also they can pass through both ordinary matter, and their own kind of matter too without noticing it.

So though you could have a dark matter planet possibly, the Earth would only notice it through its gravitational effects. But it would also be very hard for it to form, because our stars and planets formed not just through gravitational clumping but also friction between atoms and then between small pieces of matter hitting each other.

If you have none of that, it is hard to see how you could get much of it gathering together in one place, because there is nothing to slow the matter down. So it seems more likely that you only have something much more like a fuzzy thin “dark matter cloud” than anything like a planet or a star.

So, then you have two kinds of "dark matter" either hot or cold. Hot means it travels very quickly, like neutrinos not far off the speed of light. Cold means it travels somewhat more slowly.

Anyway various observations of the speeds of stars around galaxies, and galaxies in clusters suggest that there may be a lot more matter in the galaxy than there should be, and a lot more than is visible to us. You can estimate how many atoms formed in the Big Bang, and the universe seems to be too heavy for all the matter to be ordinary “baryonic matter” such as what we are made of. Indeed if this reasoning is correct, nearly all matter is “dark matter” invisible to us.

So, it is “dark" not in the sense of absorbing light, but rather, in the sense of not interacting with light at all. Not visible to our eyes or any of our instruments, or not easily. Dark matter may still be possible to observe if there are lots of it, as for the neutrinos. If you have enough neutrinos even though they can go through a light year of lead, half of hem, still a few will interact with matter. So we might observe dark matter. There are various searches underway to try to find it.