Looking out into the night sky, we see pinpricks of light surrounded by darkness. Everything that we see in the universe, including stars, planets, and plumes of interstellar gas comprise only 5% of the universe. There is a vast hidden universe of dark matter and dark energy that is responsible for guiding the evolution of the cosmos itself, from forming the structures of matter to driving an accelerated expansion that will determine the ultimate fate of the universe.

The É…CDM cosmological paradigm captures the physics that describes how the universe has changed over cosmic time. In the first tiny fraction of a second after the Big Bang, the universe underwent rapid, accelerated expansion called cosmic inflation during which the seeds of the structure that was to come were created. The universe continued to expand and cool, evolving from a universe filled with light in the earliest moments, to the epoch of galaxy formation driven by dark matter, to the current era of accelerated expansion driven by dark energy.

While we do not yet understand the underlying nature of dark matter and dark energy, or the specific physical processes driving cosmic inflation, we do know that they are not described by the subatomic structures in the Standard Model of particle physics. Answering some of the deepest questions about particle physics itself requires detailed studies of cosmic evolution, revealing the underlying nature of dark matter, dark energy, inflation, and other particles in the universe that might have played a role in driving cosmic evolution.

To illuminate the hidden universe, our efforts are focused on two main science drivers:

Illustration by Abigail Malate for U.S. Particle Physics

 Determine the Nature of Dark Matter

The gravitational evidence for dark matter is overwhelming. We have many ideas for what dark matter could be, with a handful of particularly compelling candidates with viable cosmological histories. The number of strong candidates inspires a multifaceted campaign to determine the nature of dark matter, leveraging underground facilities, quantum sensors, telescopes, and accelerator-based probes.

Illustration by Abigail Malate for U.S. Particle Physics

 Understand What Drives Cosmic Evolution

The evolution of the universe has been determined by physical processes not described by the Standard Model of particle physics, from the exponential expansion called inflation during the first moments of time, to intermediate periods dominated by radiation (potentially including unknown light species) and dark matter, to the cosmic acceleration of today. Measuring the growth of cosmic structure and the expansion history of the universe through multiple complementary methods offers unique explorations into inflation and dark energy, while also yielding insights into neutrino properties and the possible existence of cosmic relic particles from the earliest moments of the cosmos.