On dark orbit i got premium membership but i never bought it

That is simply a 'Business Tactic' from the makers of Dark Orbit.

If you don't play Dark Orbit for 1-3 Months then they give you free Premium so that you start playing Dark Orbit again.

Similar to World of Warcraft where they give you free 7 days Membership is you didn't play for 1-3 Months.

Hope that answered your Question!