The big bang may be the origin of time, therefore, it may not make sense to ask what happened before the big bang. However, this isn't completely verified. What is verified is that the big bang is the furthest point back in time of the entire universe that scientists know about currently. Therefore, simply put, scientists don't know what happened before the big bang if such question makes sense.
Nothing is not anything/a single thing; whether it be the absence of a unicorn or the absence of everything. Under the logical tautology "a or not a", nothing either is or isn't. If it isn't is its everything other than is, which is isn't. Is or not is. If it isn't, it is not a thing.