What is outside of the simulation?
Or was there ever a time when it wasnt a simulation?
Prolly a world that looks a lot like this one. Just think of the sort of video games we make now in this reality (whether its GTA or The Sims). We try to create virtual worlds that are as close to the one we live in as we can. And I'm guessing that the reality outside this simulation looks a lot like the world inside the simulation. The only differences will probably be the underlying physics.
For example, in video games the computer will take shortcuts to save processing power. Like if your character isn't inside of a building, the computer will not use resources to render the inside of the building. While in the "real world" its presumed that the inside of the building exists independent of whether someone is looking. What's interesting is that if you study the physics of "our reality" you'll learn that quantum mechanics describes our world as operating a lot like the worlds inside video games. In that when a conscious being isn't looking, particles behave differently. When we are watching, atoms behave like discrete objects. However, when we are not looking they behave like probability waves. This is called "wave/particle duality" in quantum mechanics. Its the phenomenon that was discovered in the famous "double slit experiment".
Please note that there are many other physics phenomenon in "our reality" that only make sense if we are living in a simulation. This is only one of them.