Can I use a toggle bolt if there's no space behind the drywall?
I don't suppose you have a hammer drill? Tapcons! (my favorite concrete screws, you need 2", maybe 2.25") bushed out with 1" worth of washers. Make sure they're not home first ;) If you fall into a void after drilling into the 'concrete' they're probably cinder-blocks. If you then can't get the screws to bite, make the hole even bigger and use really long toggle bolts. If this is the case be prepared for having to do it both ways as some holes will be into the meat of the block and others into the voids.