* Nothing in the universe truly moves at a constant speed in the absolute sense. Everything is moving relative to something else. Even objects that seem stationary, like a star, are moving relative to other stars and galaxies.

* The concept of constant speed is relative. A spaceship traveling at a constant speed of 100,000 km/h relative to Earth would appear to be moving at a different speed relative to a distant galaxy.

* Light is often mentioned as moving at a constant speed. Light travels at approximately 299,792,458 meters per second (about 186,282 miles per second) in a vacuum. This is the fastest speed anything can travel in the universe, and it's constant regardless of the observer's motion.

So, to answer your question more directly, the closest thing we know of that moves at a constant speed is light. However, it's important to remember that even light's speed can be affected by gravity, causing it to bend and slow down.