How synapses are hold in place if they're not phisically attached? When two neurons connect to form a synapse, between the pre-synaptic and post-synaptic cells some space is found (synaptic cleft). How does this keep in position if there is no physical connection between the two?
There is a physical connection.

Synapses are held together by adhesion molecules, like cadherins and neuroligins/neurexins. 

Missler, M., Südhof, T. C., &amp; Biederer, T. (2012). Synaptic cell adhesion. Cold Spring Harbor perspectives in biology, 4(4), a005694.