Less energy in a given volume tells you that the universe should be expanding slower, but because it is not, what that means is that energy is somehow increasing.

One idea that I toyed around with is that dark energy is a complicated network of rubber sheets. If you stretch them, you have to provide force and because of it, the sheet gains energy. The universe expansion is much like stretching this network of sheets, which increases energy.

We didn't have enough energy to create them in a lab. That means they are heavy particles, but we cannot create them due to limited amounts of resources.

We hope that one of these new particles that may get created will be this dark energy particle.

These questions used to be just the subjects of philosophy or metaphysics. Now that we can try with scientific methods, we hope to continue the tradition.

That was a convenient picture of what we thought everything was made of. The more recent realization is that it's not the case.