Sim theory
if we are in a simulation there would be zero way to tell from within it.
if any previous civilisation had the ability to build a simulated universe the chances we are in a real universe is billions to one (there will only be one real universe with billions possible simualations running, if children can make them like sims games).
If a simulated universe could create simulated universes (nested sims) its gets even more crazy.
Any sim universe would likely be smaller than its creators universe, or lower resolution (so things could seem spooky at small scales), or have to have cause and effect limited by range. this is to presevere computing power
It would also probably have a means of reverse time dilation
so you can watch it elapse faster than your own universe.it is also possible too many nested sims could crash a given simulator like a stack overflow on a x86 based computer system we use now, or force time dilation in the child universe (its speed of light constant/cause and effect would have to slow down to a new slower speed)
the setup of our universe supports all of these things (in theory) it also seems to have a reset button the big bang, this is why even the top boffins do not rule it out, many say it's as good a theory as anything else.
but also at the same time, it's not worth worrying about, because if it is a sim, it has sim jail, sim sex, sim beer, sim football and if you can never prove it does it even matter
Holographic theory
our universe is a projection and does not contain any real matter. this is also a very big theory for many scientist
I know mate ... I know - Fuckkkk 
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