The Dark Matter skin came out with the Space Station Event and was fan made by Maxbros. It cost 350 coins in the store.
There were several unreleased Dark Matter skins that could only be obtained during Halloween 2022 if you got a Halloween Crate. The unreleased Dark Matter Skins are these, the Glowing Purple Dark Matter and Glowing Orange Dark Matter.
Screenshot from the creator Maxbros of the final skin