walls
My house is a twin built in 1920. Most of the walls are plaster, some are drywall. Knocking on these with my knuckle yields a hollow sound, like tapping a door or window. However the center wall that divides our house is extremely hard, and when I tap on it, there is no sound whatsoever; except for some guitar sounds (mostly low strings) I can't hear a single thing going on next door.
What is this material? It runs down the middle of our entire house. To my eyes it's just a painted white wall!
You can use Backer Rod to fill in the crack if it is not too wide. Backer rod is an easy to compress rope-like celled foam and should be easy to remove at a later date. It is mainly used to fill deep cracks before caulking to give support to the caulk.
Backer rod comes in a variety of diameters and should be able to be found at your local building supply store.
You can buy an inexpensive asbestos test kit if you're concerned. One of my family members bought an old house and successfully used "asbestos test strips" from the local home warehouse store to determine that the floor tiles were asbestos.
Best Answer
Duplex/twin - probably a firewall, or "party wall" in the "old-shool, solid masonry" sense. Depending on the locale, sometimes a LOT of solid masonry (like 2 feet/600mm.)