Read more about solids, liquids and gases in our study guide about states of matter. In a system, the energy of the particles, which can be atoms or molecules, is known as the internal energy.

The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent). Scientists have ...

Dark energy and dark matter refers to the unseen energy and matter components of the Universe. Dark matter is invisible, non-baryonic matter hypothesized to explain phenomena including ...

Read more about the states of matter in our study guide about the particle model and heating. The energy stored in the particles in a substance changes when the substance is heated. This can ...

Our current cosmological model—known as lambda cold dark matter, or ΛCDM—relies on hypothesized dark energy to explain the accelerating expansion of the universe. However, one competing ...