A 55

If you know (or can guess) the location of a creature which you cannot see, you can attempt to hit them with any spell requiring an attack roll (touch spells, rays, orbs, etc.) with a 50% miss chance. You cannot, however, use a targeted spell.

Some spells have a target or targets. You cast these spells on creatures or objects, as defined by the spell itself. You must be able to see or touch the target, and you must specifically choose that target. You do not have to select your target until you finish casting the spell.
(emphasis added)

In related news, you cannot actually cast magic missile "at the darkness."