The entire observable universe, of which the Earth is a very tiny part, contains matter in the form of stars, planets, and other objects scattered in space, such as particles of dust, molecules, protons, and electrons. In addition to containing matter, space also is filled with energy, part of it in the form of microwave radiation.

Inertia, Mass, and Acceleration

Matter itself has energy, called "rest energy." What distinguishes matter-energy from other forms of energy is that all matter has inertia and is subject to the force of gravity when at rest as well as when in motion. Inertia measures the resistance of an object to being accelerated by a force, and the inertia of an object at rest is proportional to its mass.

According to a law of physics first formulated by Isaac Newton and later modified by Einstein in his general theory...