What is that? A black cloud?

I started playing tekkit today and suddenly saw this:

what is this?

That's an inactive rift from a (pocket dimension) dungeon from the Dimensional Doors mod by StevenRS11. If you place a Wooden Warp Door on the block just below it you'll make it permanent and be able to enter it. One-way exits of such pocket dimensions also look like this, but you would know if you created one.

Make sure you have some way to come back in case you'll destroy the exit door on the other side - like a Mystcraft Linking Book - else you'll have to cheat your way back using /tpx 0 or similar commands.

You can use a Rift Remover to get rid of it. Equip the remover and right-click on the black cloud for it to disappear forever.