The visible universe-including galaxies,stars and planets only make up 4 percents of the universe (REALLYYYY). What about another 96 percents of it? In reality, another 96 percents consists of the thing that is unseen. Besides, we can’t really identify or perhaps understand what that 96 percents is made up of. From the perspective of the scientists, they called it as a dark energy or dark matter. Why is it called as a dark matter/dark energy? (Say yes if you want to know more…kidding)
Dark matter is an unidentified type of matter and it does not interact with electromagnetic radiation (i.e. light), hence make it invisible to the electromagnetic spectrum. On the other hand, dark energy is basically an unknown form of energy which is have a direct relation with the accelerating rate of expansion of universe. However, it still remain as a hypothesis for the scientists.
According to the standard model of cosmology, dark energy contributes 68.3 percents of the total energy in the present observable universe. The mass–energy of dark matter contributes 26.8 percents while ordinary matter contributes 4.9 percents. If I were to explain these dark matter and dark energy,it is going to take forever….(I’m not even a scientist but probably will).
How do the dark energy have impact on the acceleration rate of the expansion of the universe? Dark energy has a unique property which is it has negative pressure which is distributed relatively homogeneously in space.
From this equation, it yields out a constant value for w (cosmic acceleration) thus prove that the universe is expanding with an accelerating rate. The derivation of this acceleration is shown in the link below.
The main point is we can work through a lot of mathematical formulas and do some estimation on the gravitational pull of these entities. However, there is no guarantee of proving what the unseen or so called invisible part of our universe is made up of but we do know that they are exist. Although it still remain as a hypothesis, the majority of the universe may still remain as a mystery. I provided a video in the link below which help to explain the dark matter and dark energy in a more intuitive way.