There are several free universe simulators available that offer similar features to Universe Sandbox. Here are a few options you can explore:
SpaceEngine: SpaceEngine is a popular free space simulation software that allows you to explore and navigate a virtual universe. It provides a vast database of stars, planets, galaxies, and other celestial objects, offering a realistic depiction of the universe. SpaceEngine lets you travel across space, land on planets, and even create your own spacecraft. It is available for Windows, Mac, and Linux.
Celestia: Celestia is a real-time 3D space simulation software that lets you explore the universe in a highly detailed manner. It includes a comprehensive catalog of stars, galaxies, planets, and other celestial objects. Celestia allows you to travel through space, visit different objects, and simulate astronomical events. It is open-source and available for Windows, Mac, and Linux.
Stellarium: While not strictly a universe simulator, Stellarium is a free and open-source planetarium software that offers a realistic depiction of the night sky. It accurately simulates the positions and motions of celestial objects, including stars, planets, and galaxies. Stellarium allows you to explore the sky from any location on Earth and at any time. It is available for Windows, Mac, and Linux, and there is also a mobile version for iOS and Android.
These simulators provide different features and focus areas, so you can choose the one that aligns with your specific interests in exploring the universe.