Was it as a result of some time experiment done in Shurima by some time master 
? Because the void seems like a place where time stays still but creatures exist. Kog'maw is always gonna have a "daddy", yet he's still the mouth of the Void.
Or is it something else?
I doubt it was artificially created,I think it was always there. I dunno how you get the idea that it might be connected to shurima or Zilean,these places have no known connection to the void(except if you count Vel'koz and his buddies messing up the ruins of urtistan),nor do i know where you get the idea of time not progressing in the void,there is nothing hinting at this(I mean Kog'maw is said to be still in his larval stage,how could he be in an evolutionary stage if he doesn't grow at all in the void?)
Alternate plane of existence, outside the plane that Runeterra occupies. Filled with monsters, hungry to consume the world in various ways. These types of places are rarely created by mortals - they're part of a larger universal order that goes beyond what we understand of reality.
Horror writers use similar elements. See. Arthur Machen, H.P. Lovecraft, Stephen King, etc.
The whole place is probably like a giant cosmic predator, feeding on other universes - each Voidborn that slithers its way into Runeterra is like an injection of poison into its 'prey' (i.e. Runeterra), weakening it more and more and allowing for more Voidborn to get through. It's only going to get worse...
