What is Dark Energy?
It is a hypothetical term coined to give valid logical reason to explain the accelerated expansion of the universe. As otherwise it would be impossible to explain why universe is expanding at an accelerated rate, theories like General Relativity do not say anything about it, Quantum Mechanics is quiet about it and string theory does not give any valid logical reason for it.
Source of this energy is unknown and type of energy it is also unknown thus it's called dark, dark here denotes unknown, something like you are in a dark room as you can't see what is going on.
The way we see universe it seems like it started in a big bang, expanding constantly from source moving outward toward unknown universe, this constant expansion of universe gave us idea that everything had to be connected together some time ago, we have estimated this time based on scientific calculation to be 13.8 Billion years.
Thus our view that universe started from a central bang and everything is expanding around it gives us the idea that there should be something that is pushing the universe apart at an accelerated rate, but that is only possible if there is some unknown energy that is pushing universe, one galaxy from another away.
We do not see this expansion within galaxies.
Looking at it from a different angle gives a different picture where accelerated universe is normal outcome of what is going on and how the universe started and why is it expanding and most importantly where it came from.
I would prefer to call big bang as big splash, problem with big bang concept is that universe started with a size of an atom or less and expanded into this large domain, which sounds very improbable and does not make logical sense as to what mass of whole universe was doing in an area of single atom, then it expanded beyond trillion times it's size, including dark matter and all the black holes, stars and planets around those stars?
I will present to you a simpler explanation that not only explains the accelerated expansion without the use of word "Dark" but also explains you where all the matter came from and why are we still expanding and where it would all end.
To see how it works, let us first see how gravity works on an object.
Imagine two object dropped toward a huge mass, one dropped at t and other dropped at t+1, both masses would travel toward the object accelerating toward it with a, at every instant the distance between both object will increase. This would happen because gravity isn't constant but changes with distant, thus first object will always have same higher amount of gravity acting on it, while object that is further away will experience less gravity thus would accelerating slower, but acceleration at point d for both objects will be same.
But because first object is moving faster with time, the distance between both objects will increase due to difference in acceleration.
To simply check what I'm saying we could do simple calculation using formula for calculating distance travelled in time if g is constant.
s = ut+1/2gt^2
Here we consider u = 0, thus we are only left with s = 1/2gt^2.
First body is dropped 1 second before the second body.
Time D1 D2 Difference
0 0 0 0
1 5 0 5
2 20 5 15
3 45 20 25
4 80 45 35
We can clearly see the distance increase if G is constant, note G really isn't constant for both bodies, body nearer to the mass has higher G acting on it.
If you are body 2 you'll see that body 1 accelerating away from you, but because you could see the ground, you know you are being attracted toward it, if you can't see the ground because information you have is limited you'll think there is dark energy that is pushing both bodies apart.
If we just add another super massive body, we know where is this energy coming from and we are accelerating toward it.
Explanation of where all this matter came from became easier too, it came from the super high mass black hole, that we just bubbled out of, like water, where each drop is moving toward ground but holding together because particles pull toward each other. I will explain in further chapters what gravity really is, clearing that part out.
Thus our galaxies hold together just like those drops that splashed out of the super massive body and it's moving toward it where galaxy 1 is faster than galaxy 2 and due to changing of G and time difference of body moving back we see universe expanding at an accelerated rate.
That solves the problem is dark energy, but why did the big splash happen? For that you have to understand a bit about my theory of atoms that applies to atoms as well as everything else, this is not a quantum theory but a theory that takes into account micro and macro levels.
I may not even be smart, maybe this is why I came up with something that is easier to understand and logical because surely I can't understand logic of Quantum Mechanics and surely not understand logic of String Theory.