An invisible form of matter that does not emit light but exerts gravitational pull, holding galaxies together.

Evidence for this theory is overwhelming, most notably the radiation. Discovered in 1964, the CMB is the "afterglow" of the Big Bang—faint radio signals coming from every direction in the sky, representing the moment the universe cooled enough for light to travel freely. 3. The Composition of the Cosmos

If the universe is expanding today, it must have been smaller and denser in the past. The suggests that approximately 13.8 billion years ago, the entire observable universe was concentrated in a state of infinite density and heat called a singularity.

Cosmology shifts our perspective from the local to the infinite. It reveals a universe that is dynamic, evolving, and largely invisible. While we have mapped the history of the cosmos back to its earliest seconds, the nature of dark energy and the exact mechanism of the Big Bang remain the great frontiers of modern science.

A repulsive force that appears to be accelerating the expansion of the universe, pushing galaxies apart at an increasing rate. 4. The Fate of the Universe