Empty space will appear black because of the lack of particles that can be found in that area. There was a time when people thought that molecules were the smallest particles that can be found in this world.
As time went by, they realized that atoms are far smaller. Then, people realized again that neutrons, electrons, and protons exist. People have assumed that they would be the smallest particles until quarks were discovered. Empty spaces may not hold any particles.
Particles usually show their presence by giving off light and so much more. In space, there are still a lot of empty spaces, especially since there are reports that the universe is continuously expanding.
The empty spaces that we might sometimes see when we look at the night sky may appear black. With the use of different tools and machines, you would see that the empty spaces may not be so empty after all. There are a lot of heavenly bodies that can be found in the sky. Most of them are just not visible to the naked eye.
The real empty space does appear black, though, because there is nothing there that can make the area light up. Even if a heavenly body does not have its own light, there are some, like the moon, that will be able to absorb artificial light from a nearby source and light up. Empty spaces will not contain anything, so it will appear dark.
It may seem that this question is very easy to answer, but some people are unable to come up with an explanation for this even if they have tried their hardest to understand space. The sky can be blue or white during the daytime because there is enough light available that will reflect on the sky. During night time, there is no bright source of light.
The moon can somehow give its own light, but it will only illuminate a certain area. This explains why the area around the moon is not completely black, while the rest of the sky is black in color. There are still a lot of experts that are trying their best to provide insight regarding this question to make it even easier to understand.
When people look up the sky, they will rarely see it empty. Even if there are no stars available, there may be clouds that will be covering up the other heavenly bodies. This is what people see when they do not use tools. Professionals who use different powerful equipment will be able to see empty spaces. These empty spaces are sometimes called black holes.
What they do is they feed on the different particles that are found within the area. The problem with the black hole is the more that it is fed, the bigger that it becomes. Right now, this is still considered to be a theory. There are some reports that black holes exist, but there are just some people who do not believe it.