Countering dark swarm is tough. That's the great thing about dark swarm, 1 little defiler can cause the enemy to go all out to counter it. Anyways, if you're protoss, your best bet is templars or reavers. Both do great splash damage. Zealots also do good, but 4-6 lurkers under dark swarm covered by 10-12 hydras are nearly impossible for zealots to really hurt. Just keep templars by your base at all times for things like this. Now, if you're terran, it's a bit more tricky. You should always keep a science vessel around to irradiate the zerg, that will really hurt them. Tanks are also very good (sieged) since they cause major splash and dark swarm does nothing to stop tanks. That's about it. Nothing else will really beable to touch dark swarm. Forget all the high stuff like BC's and carriers..... now in a zerg vs zerg battle, best way to counter it is to use it yourself Lurkers also work magic when it comes to dark swarm.
oh and to lilihaha....... Dark Swarm is an ability that defilers can use that prevents non-melee units (IE: hydras, dragoon, goliaths, battlecruisers, wraits, ect.) from attacking anything that is under the dark swarm. It's very useful.