When we refer to space as "dark," it's not in the sense of complete absence of light or electromagnetic radiation. Rather, it means that space appears dark to our eyes because it lacks a significant source of illumination like the Sun. Let's explore this concept further.
In the vast expanse of space, there are indeed numerous sources of light and electromagnetic radiation. Stars emit light, and galaxies, nebulae, and other celestial objects also emit various forms of radiation across the electromagnetic spectrum. However, the distance between these objects is immense, resulting in a significant amount of empty space between them.
When we observe space from Earth or with telescopes, the light from distant stars and galaxies has to travel through this vast expanse of empty space before reaching our eyes or instruments. Along this journey, the light encounters various obstacles, such as dust, gas, and other intervening matter. These materials can absorb or scatter the light, causing it to lose energy or change direction.
Furthermore, the expansion of the universe causes a phenomenon known as cosmological redshift. As light travels through expanding space, its wavelength stretches, shifting it towards longer, redder wavelengths. This effect is more pronounced for distant objects, making their light appear "redder" and more difficult to detect, especially at higher distances.
Combining these factors, the light that reaches us from the distant reaches of the universe becomes extremely faint and often difficult to detect directly. While space is not completely devoid of light, the overall brightness is much lower compared to the light we experience on Earth, which is primarily due to the Sun's proximity and its intense illumination.
So, while space is not completely dark in the absence of light, the vast distances, the absorption and scattering of light by intervening matter, and the redshift of light from distant objects collectively contribute to the perception of space as being "dark" when observed from our vantage point.