• A galaxy is a huge collection of gas, dust, and stars. Earth and its solar system are part of a galaxy called the Milky Way.

• Most galaxies are organized into galaxy groups or clusters. The Milky Way is part of a group of galaxies called the Local Group.

• All of these systems are held together by a force called gravity. Gravity is an attractive, or pulling, force that exists between all objects in the universe.

• The force of gravity between two objects depends on their masses and the distance between them. The greater the masses and the shorter the distance, the stronger the force of gravity.

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