Ethan Siegel in Forbes:
No matter how much we might try and hide it, there’s an enormous problem staring us all in the face when it comes to the Universe. If we understood just three things:
1. the laws that govern the Universe,
2. the components that make up the Universe,
3. and the conditions that the Universe started off with,
we’d be able to do the most remarkable thing of all. We could write down a system of equations that, with a powerful enough computer at our disposal, would describe how the Universe evolved over time to transform from those initial conditions into the Universe we see today.
Every single event that occurred in our cosmic history — to the limits of classical chaos and quantum indeterminism — could be known and described in great detail, from the individual interactions between quantum particles to the largest cosmic scales of all. The problem we face, when we try to do exactly that, is that despite all we know about the Universe, what we predict and what we observe don’t quite match up unless we add at least two mystery ingredients: some type of dark matter and some type of dark energy. It’s a remarkable puzzle to solve, and something every astrophysicist has to reckon with. While many love to present alternatives, they’re all even worse than the unsatisfying fix of dark matter and energy. Here’s the science of why.