My theory relies on accepting a few things before we start.  That we are living in a Multiverse with the possibility of an infinite number of universes.  That dark matter and energy do exist.  On a quantum level that Superstring Theory is correct and some sub-atomic particles can be in several (or possibly everywhere they have been and ever will be) at the same time. 

Lastly that something I read or heard so long ago I can't remember any other details is also correct (probably something to do with the theory of evolution).   That the primary function of a living organism is to multiply itself as many times as possible.  To give itself the best chance of survival in an evolving environment.

If you can accept all that my theory is pretty simple.

Our universe is a living organism something like a vast amoeba.  It lives in the Multiverse..  Dark matter and energy are the portion of the parent universe still in the previous universe that ours is in the process of splitting off from.  Which is why we can't detect them, they're not "here" to detect.

The Big Bang was the beginning of this split as the other universe divides itself in two.   The galaxies, stars and planets are akin to its constituent parts (nucleus etc.).

My theory would also explain why our universe is expanding at an accelerating rate.  Surely it should remain constant if it was formed in a single explosive event in a vacuum.  But this is explained if the division is not complete and as more of this universe is split off  "the push" away from the parent gets stronger.

I think my theory explains the formation of our universe pretty neatly using the scientific observations and theories available at present but of course still leaves another question.   Where did the amoeba come from?