According to our current understanding of physics, the speed of light in a vacuum is considered to be a fundamental constant of nature denoted by the symbol 'c.' The special theory of relativity, developed by Albert Einstein, introduced the concept that the speed of light in a vacuum is an absolute speed limit and that no material object with mass can reach or exceed this speed.
In the context of objects moving in space, if an object with mass were to approach the speed of light, it would require an infinite amount of energy. As an object accelerates, its energy increases, and as it approaches the speed of light, the energy required to continue accelerating becomes prohibitively large. This means that massive objects cannot reach or exceed the speed of light.
Furthermore, as an object with mass approaches the speed of light, its relativistic mass increases, and the amount of energy required to further accelerate it also increases. This phenomenon is described by Einstein's mass-energy equivalence principle, which states that mass and energy are interchangeable. The increase in relativistic mass leads to an increase in the object's inertia, making it more difficult to accelerate further.
On the other hand, massless particles, such as photons (particles of light), travel at the speed of light in a vacuum. Since they have no rest mass, they are not subject to the same limitations as massive objects. Photons always move at the speed of light and cannot move slower or faster.
Therefore, the velocity of a body moving in space, including light, remains constant at the speed of light in a vacuum because of the fundamental properties of mass, energy, and the constraints imposed by the special theory of relativity.