Why did dark energy overcome dark matter to create the expanding Universe?

The density of dark matter decays as ordinary baryonic matter while dark energy has constant density. Eventually the latter would dominate to drive accelerated expansion.

 

There is no known relationship between dark energy and dark matter.

 

It didn't.

The dark matter in our universe was never enough to make our universe recollapse on itself, and the first few billion years of our universe experienced essentially no impact from dark energy. What dark energy has done is make the expansion rate over the last few billion years higher than it otherwise would have been. This is because the dark energy doesn't dilute as the universe expands: its energy density stays the same.

 

It's just not at all true that dark matter and dark energy are opposing forces that were close in strength. And Einstein certainly never said it (as the term dark energy wasn't coined until after his death).

Einstein's original idea of a cosmological constant was conceived because Einstein thought the universe should be static. He used the cosmological constant to balance the matter (dark matter wasn't known at the time, either). The idea there was that matter tries to fall inward, causing the universe to collapse. The cosmological constant pushes outward. So if you have a matter density and cosmological constant that are perfectly-balanced, then you can have a universe that neither expands nor contracts.

The problem with this idea, besides just being incorrect, is that it is unstable: a universe sitting in this configuration is just at the boundary of collapsing into a singularity or expanding forever. All it needs is for a little bit of matter in one region to migrate to another, and the whole system destabilizes.

So this idea of dark energy balancing dark matter was never a viable model. It wasn't a good idea to begin with, and it certainly never described our universe.

 

Einstein didn't come up with the idea of dark matter, he came up with the cosmological constant previously mentioned. That'd be dark energy. And you are completely forgetting inflation.

 

You've still got a rather confused notion of what dark matter and dark energy do.

Again: dark energy never "overcame" dark matter in any way, shape, or form. Neither dark energy nor dark matter is responsible for the fact that the universe is expanding. What they do do is impact how the rate of expansion has changed over time.

For most of the history of our universe, when it was expanding the fastest, dark energy was completely irrelevant. The gravitational attraction of matter (normal and dark) slowed the expansion dramatically. When the matter density became small enough a few billion years ago, the constant density of dark energy became relevant to the expansion, essentially putting a floor on how low the expansion rate could go.

It's entirely relevant, because your question makes no sense at all when applied to our universe.

 

I thought dark energy was, in the form of the cosmological constant; at least that's how it's worded in numerous articles. Did I get this right?

 

Bear in mind that matter and dark matter were actually winning the gravity battle until about 5 billion years ago. Expansion was actually slowing down, as expected, up to that point in the history of the universe. Dark energy gained the upper hand at that time and expansion has been accelerating ever since. This was completely unexpected when discovered by Perlmutter and Riess at the turn of the century - who were actually seeking to determine if and when the universe might ultimately recollapse. This mysterious repulsive force got saddled with the unfortunate name 'dark energy' which, of course, has proven confusing.

 

To be fair, some theorists suspected a small cosmological constant prior to the observations by the teams led by Perlmutter and Riess, but it is true that few people thought the idea likely. Here's one example from 1995, and I'm sure there are earlier examples as well:
http://arxiv.org/abs/astro-ph/9504054

 

There are several threads which discuss the precise meaning of EXPANSION

The fabric of space-time is NOT expanding

To quote Lee Smolin,

What exists, is a single fabric of space-time. The whole entire fabric, from the Big Bang to Big Crunch, exists. We confuse ourselves by focusing on space-like SLICES of the fabric which we "now". The whole entire fabric exists, with no more distinction between yesterday and tomorrow, as between one place and another.

Thus, SPACE-TIME is NOT expanding. As our sense of now passes through time, we move along our 3+1D world-lines like trains on rails. And those rails are taking us from smaller space-like slices of space-time, to larger ones.

Imagine a vast fleet of ships encircling Antarctica. As they steam northward, towards the equator, the distances between them seem to increase. But earth and the oceans are NOT changing size in any way. And in GR, the whole course of the ships, i.e. their world-lines, are preplotted. Nothing is changing, not the ships courses, nor the earth...

Not worldlines of particles, nor the fabric of space-TIME.

Dark energy and mass-energy all have positive energy density. They affect the GLOBAL topological shape / form/ structure of the fabric of space-time in different ways... Mass and energy INSIDE of space-time cause it to close through time, like lines of longitude on earth which converge towards the north pole...

Dark energy causes those lines of longitude to curve the other way, diverging outwards so as to form space-time into the shape of a vase or bowl.

You don't need to consider negative pressures, or anything pushing anything else...
The whole fabric of space-time is, and different kinds of energy densities affect its global topological structure differently. For discussions of standard GR, time is an illusion, and we confuse ourselves by speaking of anything stretching, expanding, changing. Space-time is shaped such that space-like slices of it are larger at later coordinate times, of clocks on world lines threading through the fabric. But the fabric is fixed and is not changing.

You can stack big plates on small plates... That doesn't mean any plates are stretching, expanding, growing

 

One of space's properties is that it can expand, and that means it contains Dark Energy, so (don't take my word on this as a complete answer) the more space there is the more Dark energy there is because dark energy lets space expand but Dark Matter is not necessarily found in empty space so Dark energy dominates in this case. Does that answer your question?

 

But Dark energy allows for it and this is one case. But how can space expand without dark energy or the cosmological constant? And I meant Dark Matter, it was a typo, by the time you read this I would have fixed it.