Hi /u/AdiSoldier245!

I think the best way to understand, why photons cannot escape from within the event horizon, is the following way:

According to General Relativity, gravity is the curvature of spacetime. The stronger the gravitational field, the more extreme is the curvature of spacetime. Inside the event horizon of black holes, the curvature of spacetime is so extreme, that no path through spacetime exist for photons, which lead anywhere else but to the singularity at the center. That is, spacetime itself is curved so much, that there is, physically and geometrically, simply no path that light can follow to the outside.

Yes, that is true. Thanks!

Hi /u/Pristine_Pop_7818!

The answer is simply, that energy is not conserved in our expanding universe.

Noether's theorem tells us that any quantity which has a continuous differentiable symmetry in the action has an associated conservation law. That is, for example, the translational symmetry of the universe is associated in a one-to-one correspondence with conservation of momentum.
This also tells us, that time-symmetry is associated with conservation of energy. As our universe is expanding, time-symmetry is broken. Thus, Noether's Theorem tells us, that energy is not conserved in our universe. In practice, this means that dark-energy density is constant. Hence, as space(time) expands, dark energy is created.

Hi /u/Atharaphelun!

The sidebar states:

>LI5 means friendly, simplified and layperson-accessible explanations - not responses aimed at literal five-year-olds.

Hi /u/Drippidy!

In order to explain this fact, we have to understand binding energy:

Probably the most famous equation in physics is E=mc². It tells us that mass is a form of energy and can, therefore, be transformed into other forms of energy (just like p.ex. movement energy can be transformed into thermal energy).

Atomic nuclei are made up of protons and neutrons. Protons hold positive charge and therefore repel each other. The reason why atomic nuclei can nevertheless be stable is, that they are held together by short-range attractive forces called the strong force and the weak force. If this sound confusing, your take-away should be that there are two kinds of forces in atomic nuclei, one kind is attractive and the other is replant. This pull-and-push game means, that there is one combination of protons and neutrons that form the most tightly bound nucleus: Iron. In iron, the attractive forces win against the repellent force by the largest margin, so to speak, forming the most tightly bound nucleus. All other combinations, i.e. nuclei that have both fewer or more protons and neutrons in the core, are less tightly bound than iron.

Therefore, very light nuclei, which have fewer particles in the nucleus than iron, get more stable by gaining protons and neutrons, while nuclei that are larger than iron get more stable by losing protons or neutrons. In physics, being stable is always associated with minimizing your potential energy, so the closer nuclei are to iron, the lower is their energy level. As iron is the most tightly bound nucleus, it is the most stable configuration. Therefore, fusing iron into heavier elements requires considerable energy to be put into the system, rather than gaining energy through fusion as is the case for lighter elements.

Hi /u/ppyrosis2!

It is a common misconception that the universe expanded from a single point in the moments after the Big Bang. If the universe is infinite, it has always been infinite; and if the universe is finite, it is closed. Either way, the universe did not start in a single point in the moment of the Big Bang, but rather the entire universe expanded at once.