Well we know a few things about the Void for certain, given indications from the most recent void related content we've seen, which is mostly in the way of Rek'Sai, Vel'Koz, Baron Nashor, the Zz'rot Portal and the Rift Herald. What are those you ask? Well...
- The Void is dark. I know, I know, but still, it's something!
- The Void has some physical component to it, as it's been confirmed (if I remember rightly) that Rek'Sai's burrowing nature wasn't an adaptation for Runeterra, and the same applies for the Xer'Sai.
- The Void is directly linked to Runeterra, with the effects being most visibly noticable in the Sai region (Where Icathia most likely is) and on the Summoner's Rift. Both of these locations have enough of a connection to the void that portals can open there naturally, allowing void creatures to enter Runeterra and, with recent revelations of what happens to a void-infused scuttle crab, Runeterran organisms to enter the Void. Also before you say anything, Kassadin's never entered the Void as far as we know; he was transformed in Icathia specifically.
- The Void has its own specific energies that are integrated in to the biology of its native inhabitants. We've always known humans had the potential to tap in to this power but only in more recent developments have we seen Void entities truly utilizing these powers themselves.
- Certain Void entities will return to the Void upon dying, implying a deeper connection to their home more than just being residents.
Beyond this details about the Outside are sketchy, but I'll stand by my belief that the Void is a world (or maybe multiple worlds) of a physical nature that exist in a dark place. Given the name "The Void" is so commonly used but it's seemingly teeming with life it stands to reason the void is one of light in this place, implying a potentially dark universe with no stars, filled with a different energy that allows for life to still exist, but in a very different way to Runeterra.
I also stand to the idea that the Void energy used by the Voidborn is the same as the Void magic used by Malzahar and Kassadin, and that organisms from the Void are just naturally magical beings. I'll extend this by saying it's this personal magic that allows individual void creatures to invade Runeterra but not the other way around; magic manifests differently in Runeterra and travelling between worlds requires a lot of portal magic to accomplish, something which is incredibly difficult to perform for just about anyone, since most living organisms on Runeterra aren't inherently magical and have to get it from different sources.
I can go full WoW here just to throw an idea out, but I'm not 100% sold on it myself. In WoW there's a thing called the Twisting Nether which connects all worlds and is highly magical. The demons of the Burning Legion inhabit it and use it as a means of invading worlds so that they can destroy them, but the Legion still has entire worlds that they inhabit such as Mar'duun and Argus. Going by this example the Void could be a nebulous aspect of the universe that links planets, but for some of these worlds they get sucked in to the Void or just permanently linked to it, forever darkening them and infusing them with the energies of this mysterious thing. The Void links all worlds so with enough provoking from one side or the other the Void can begin to interfere with other worlds, resulting in invasions and the conversion of entire planets in to the Void.
Alternatively a different angle could be that the name "The Outside" could be very literal, and the Void could be a place on the outermost edge of the universe Runeterra inhabits, in some nebulous zone that is technically outside of the universe, but still connected to it.
God damn I've said Void so many times there but you get my point. That could all be bollocks but hey, it's fun to think about!
and 
have both said that the daylight on Runeterra is too strong for them. Creatures such as the Rift Herald can live/come from the Void, but it's also possible for human beings to be touched by the Void and acquire otherworldly powers, as is the case with 
and 
.
