As others have already written, it could be a beam or a metal bracket or something similar. But you simply don't know what or how strong this possible material is there, wood, metal, or whatever, which is why your question simply cannot be answered. Depending on the material and the chosen drill bit, you could drill for an hour without anything happening. In this case, I would definitely look for a spot where I can get through "easily." If the wall structure is like that, you might better look for a spot where, after an initial drill, you can then poke through with a wooden stick or something similar to the other side to make sure there's nothing in between that you better not drill into.