When you shine a flashlight in space, the light emitted from the flashlight will continue to travel indefinitely, unless it encounters an obstacle or is absorbed by a medium. In the vacuum of space, light behaves as an electromagnetic wave and follows the laws of physics related to the propagation of electromagnetic radiation.
According to the principle of inertia, light will continue moving in a straight line unless acted upon by an external force. Since space is mostly empty, the light from the flashlight will not encounter significant resistance or obstacles to alter its path. It will continue traveling at the speed of light, which is approximately 299,792 kilometers per second (186,282 miles per second).
In the absence of any significant gravitational fields or massive objects, the light will essentially continue traveling forever until it interacts with something. If it encounters a celestial body, such as a planet or star, it may be absorbed, scattered, or refracted by the atmosphere or other materials present.
However, it's important to note that the universe is not completely empty, and there are various particles and dust scattered throughout space. Over extremely long distances, these particles can have a cumulative effect and cause a phenomenon known as cosmic extinction, where light gradually diminishes in intensity as it travels through space. Nonetheless, this effect is relatively minor and only becomes noticeable over astronomical distances.
So, in general, the light from a flashlight in space will keep traveling indefinitely until it interacts with something or is affected by cosmic extinction over extremely long distances.