If you were a computer programmer and you were to create a simulated automated world, that would constantly evolve and self run after initial designs. First you would have to code the rules, laws, and core elements of this world. While this is being coded, the program is not run yet, so none of it exists. When you actually have the foundation you turn it on.
Now there is a big empty universe. Depending if you have things in place yet or not. Then comes creating genetic code. You put the first life forms in motion so the process from there on is completely automated. To save time, you copy this genetic code and build up from it. The very basic life forms were the elementary phase and you recycle what you can of the code for other life forms and variations. You program in the ability for it to self alter and adapt to conditions and change over time. The highest form has such an advanced level of AI they can become self aware and have the ability to use the resources on this world to create all manner of things. In the end you'll never physically be in their world and they'll never physically be in your world either.
What is to stop us right now from being able to create such an advanced program with AI? If we will be able to do this, how would we know if we are not ourselves part of one?