Who is the participator in volume spikes

I have kinda a remedial question on how markets work. I know in the option market there is a market maker that stays zero delta throughout and sells or buys peoples options. But what happens in stocks I often see time were ill be trading a low volume stock and the avg size will be 500 , 500 , 1000, and then ill get a huge seller come in and sell 168k Whos on the the other side of that? is somebody obligated to fill those large orders and just hedge with options or something like that?