So here is my theory behind cardassians and why we don't see them to the 24th century. Well I have 2 theorys. 1) Cardassians where prewarp with the union being confined to their own solar system and perhaps nearby stars if they had high impulse speed. It's not until they reached the mid 23rd century they got warp and began massive expansions. A) being prewarp the federation would not have made first contact. B) IF confined to there own solar system/nearby stars they could of just been missed in the vast quantity of stars. C) they could of been on the edge of federation's space and when they got warp drive and started massive expansions they could of resented the federation getting a head start and grabbing so much territory nearby, hence the border wars. Problem was this was the organians mention encountering a ship in the 22nd century. though this could have been a long range high impulse scout ship with limited interstellar capacity. 2) similiar to above but they where stuck at very low warp (like warp 1) and had no break throughs to higher war until the end of the 23rd century, limiting their expansion to local stars. It also raises the question..... their are likely myriads of prewarp civilisations with federation boarders...... what happend when they get to warp 1 and the federation's make first contact and they DONT WANT to join the federation? Will that race resent the fact they are surrounded on all sides and unable to expand?