Only about 4% of all matter in the universe is visible to us and is made up of protons, neutrons, and electrons. That is, the type of matter that makes up stars, planets, suns, living beings …
It also forms the entire table of the elements, and naturally, hydrogen, the most abundant element in the known Universe, and helium, which is the second. However, thus far, we have only explained 4% of the universe. What is the rest made of?
Dark matter and energy
All the remaining matter, that is, practically the entire universe, is invisible. We have never seen it, we have never touched it, we do not know where it is, much less have a slight idea of how to get there. We don’t know what it looks like or how it behaves. We do not know exactly what it does. All of it is pure conjecture. Specifically, this part of the universe is made of dark energy (73%) and dark matter (23%) .
Dark energy is a hypothetical form of matter that would be present in all space, producing a negative pressure and that tends to increase the acceleration of the expansion of the Universe, resulting in a repulsive gravitational force .
That is, almost the entire universe is made of something that we do not understand very well. We only know that the universe is expanding faster and faster, and that this has to be the result of dark energy, as suggested by observations of very distant supernovae by the Supernova Cosmology Project at Lawrence Berkeley National Laboratory.
Dark energy is very thin, and does not interact with the forces of the universe, except gravity. About its composition there are only a handful of hypotheses. Some theorists think that dark energy and cosmic acceleration are a failure of general relativity on very large scales, larger than superclusters. But an honest scientist, when asked "what is dark energy made of?" Should simply shrug their shoulders .
Dark energy can turn into dark matter when struck by baryonic particles, thus leading to particle-like excitations in some type of dynamic field, known as quintessence. Quintessence differs from the cosmological constant in that it can vary in space and time.
The most direct consequence of the existence of dark energy and the acceleration of the universe is that it is older than previously believed. If the age of the universe is calculated based on the current data of the Hubble constant , an age of 10 billion years is obtained, less than the age of the oldest stars that can be observed in globular clusters, which creates an insurmountable paradox.