Providing that it does not collide with another object, what is the most likely reason that an object moving through space would change velocity?

GRAVITY !

The object doesn't have to collide or even TOUCH something else in order to feel a gravitational force toward it.

If there's a gravitational force between the space object and ANYTHING else, (and there always is !), then that force will change the object's speed or direction of motion.

increasing transnational kinetic energy. because the transnational kinetic energy formula consists of only two variables, mass and velocity, increasing one of those properties is the only way to increase an object's transnational kinetic energy. increases to mass and velocity, however, do not have the same impact.