There seem to be two parts to your question
Why is the universe flat?
Why is the energy density of the dark energy component so similar to the matter density?
The theory of inflation was proposed to explain 1) as well as the 'horizon problem'. For 2) there doesn't seem to be a generally accepted and convincing answer at the moment.
Supporters of the ...
Dark matter simply refers to matter that does not emit light. In other words it does not interact with the electromagnetic field. We have never been able to observe it directly, since our telescopes can only see incoming light from the universe.
But we can infer it's existence due to the fact that gravitational lensing due to galaxies is a lot stronger than ...