Neil Savage in Nature:
Scientists have theories about dark matter and dark energy — and some observations — but both are poorly understood. Here are four of their biggest questions.
1. Is there a dark-matter particle?
Why it matters
Subatomic dark-matter particles, analogous to the particles that make up the visible Universe, would fit nicely into current physics models. But discovering that dark matter is something else would expand scientists' understanding of the Universe.
What we know
Weakly interacting massive particles (WIMPs) are the leading candidates. Other possibilities include a potentially very light particle called the axion and a more recent proposal — the very heavy Planckian interacting massive particle.
The search for the particles is ongoing. Physicists at CERN's Large Hadron Collider are looking for WIMPs, the Axion Dark Matter Experiment is running at the University of Washington, Seattle, and China has launched the Dark Matter Particle Explorer.