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Mass and energy are two faces of one same thing. Energy and mass are not two but one( something) They can be related as :

$$E = mc^2 $$

I want to ask that what decides that "something" is going to become mass or stay in the form of energy (like dark energy, or some other kind)

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This is kind of a tough question to fully address, since it is fairly broad. But the essence of the answer is that if the conditions are right to allow energy to turn into mass, we can mathematically give the probability that this will happen (these mathematics have, of course, been verified by experimentation).

For starters, as you pointed out, energy and mass are two sides of the same coin. But when jumping from one form to another, energy has to be conserved. This means you'll only get energy turning into a massive particle if there's at least as much energy in a given place as what would be in that particle according to the $E=mc^2$ relation. You can't get something from nothing, so if there's not enough energy, it won't turn into mass.

The second thing to consider is what type of energy is lying around. Different types of energy relate differently to mass. For instance, energy in the form of photons relates to mass very well; that is, it is very compatible with transforming into massive particles (so long as all the conditions are met). On the other hand, as far as we know, there doesn't seem to be any physical mechanism allowing dark energy to transform directly into matter (it's also got a more or less constant energy density which is, quite frankly, too low to produce matter in a small region).

Furthermore, one has to consider all of the conservation laws. For every particle you create, you need to make an anti-particle as well. Momentum has to be conserved within the system, as does charge (electric, colour, whatever type of charge). Momentum conservation is one of the prime reasons you can't have a random photon in free space just spontaneously turning into a couple of particles (you need another photon or charged particle nearby).

Once you satisfy all the conditions of any given situation, then you start having a chance that energy will turn into mass. Of course, the more energy you have around, the higher the chance that some of it turns into matter. But that's all it is, a chance. There's no certainty that at a given point the energy will turn into particles. You can do the math and reasonably expect to see it happen in a certain range of time or around a certain energy level, but it's all random; down to probabilities. Sometimes those probabilities are high, like when you use the potential energy from separating a quark-antiquark pair to create another pair. Other times the probability is low, like bouncing a $1.1MeV$ gamma photon off a gold nucleus; can it create matter? Yes. Will it? Probably not on the first photon.

TL;DR

You need the situation to be just right; there has to be enough energy available of the right type and conditions have to be perfect to allow for all your conservation laws to be upheld. If you have all that, then it boils down to probability. Basically random chance.

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  • $\begingroup$ Not random chance, but a prpbability calculable from quantum mechanical equations with boundary conditions $\endgroup$ – anna v Dec 1 '17 at 17:24
  • $\begingroup$ @annav The probability distribution may not be uniform, but that doesn't make it non-random. "Random chance" implies unpredictability of an individual event, not equality of probability amongst possible events $\endgroup$ – Jim Dec 1 '17 at 18:51
  • $\begingroup$ for the general reader random is what he/she expects in a lottery, a uniform probability distribution, that is why the distinction should be made, the probability outcomes are not uniform. $\endgroup$ – anna v Dec 2 '17 at 4:49
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The E=mc^2 is misleading and that is why it is not used in particle physics where one has to discuss energy and mass. Misleading because this m and this E are a function of velocity, and thus not invariant under Lorenz transformations.

In particle physics there exists the invariant mass,

invar mass

the "length of the four vector involved, which characterizes the elementary particles in the table of the standard model, and is invariant under Lorenz transformations

elempart

.You will notice that all of them are accompanied by quantum numbers.

Ensembles of elementary particles will have an invariant mass that characterizes them from the vector sum of the four vectors and the corresponding quantum numbers.

These particles ( and ensembles) obey quantum mechanical equations where the appropriate to the subject forces appear as potentials to be solved .

It is conservation laws of the quantum numbers and energy and momentum vectors on the one hand, and the possible interaction potentials in the quantum mechanical equations involved which will decide whether new particles or ensembles of particles will appear in a specific boundary condition problem or only energy momentum in four vectors describing particles and ensembles.

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