What if the Universe is expanding and contracting simultaneously? There have been numerous theories proposing that the Universe might expand, then contract, creating a cycle of existence. But, what if serial time as we know it is also a function of the expansion of the Universe, and, when it reaches its ultimate expansion point whatever it might be, when it begins to reverse course, time reverses also?
Therefore, what is perceived as the vast mass of dark matter is actually the Universe itself rushing at us downtime, at near-relativistic speed, which accounts for our perception that it is more massive than "our" Universe. A viewer going the other way would perceive "our" Universe as both invisible dark matter, and massive due to the closing velocity. "Dark energy" is nothing more than the gravitational attraction of the downtime Universe, which, so far, is attracting us outward. (Hey, gravity works across time! Who knew? Nothing else blocks gravity. We don't know what gravity is, anyway.) When the two masses pass, the respective gravitational effects will then have a braking effect, which will cause our outrushing Universe eventually to slow to a stop and begin to contract, while the downtime Universe crashes back together and explodes outward again in a new Big Bang.
As with my hypothesis of Lumpy Time, you read it here first! Speculative as it is, I like this idea because of its elegance in that new forms of matter or energy are not needed to explain the observed behavior. Your comments, especially those of the Nobel Prize committee, are welcome.