How do cosmologists supposedly simulate Dark Matter and DArk Energy when they have no idea what causes either of those phenomena?

I just glanced over an article on MSN about a new simulation for the first billlion years of the Universe, and couldn't help but laugh. They can't prove what Dark Matter is, it may be another particle or it may be something other than a particle or wave. They can't prove and have no idea what Dark Energy is, so how do they supposedly simulate a billion years worth of the Universe with NO FUCKING CLUE what those two variables represent nor what formula those forces use?

There's a motto for any computer simulation:

"Garbage in equals Garbage out."
How do cosmologists supposedly simulate Dark Matter and DArk Energy when they have no idea what causes either of those phenomena?
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