First, although I was always taught that the axon and a dendrite were separated by a synaptic gap of about 20 to 40 nm, I recently saw a reference I've since lost that seemed to suggest we've found that not all axon-dendrite pairs have literal gaps between them; some touch. Unfortunately, I didn't have time to read it, I have not recovered the source, and a search hasn't yet helped. Is there research that suggests that some axons and dendrites actually touch?
Second, what exactly is in the gap? Liquid, gas, nothing/vacuum? Of course, neurotransmitters can cross the gap, but I'm asking about whether something's always in the gap and if so what.