We know that energy cannot exist without mass and if there was no energy associated with mass, nothing in the universe would ‘work’.
The major questions then are ‘what is mass and what is energy?’

In my opinion ‘energy’ is the ‘motion’ of mass and ‘mass’ is ‘something that can move.’

Explaining Einstein’s formula: E=mc^2 then means that the total amount of ‘motion’ within (intrinsic to) any quantity of ‘mass’ can be calculated by using the said formula.
Any external force acting on this ‘intrinsic/mass motion’ is a vector force and can be calculated separately.

What bothered me for a long time about this formula was that the ‘c’ is ‘squared’.
The ‘c’ is a constant that is derived from the speed of light, but the speed of light is supposed to be the ‘speed limit’ of everything in the universe, what is the reason for multiplying the ‘speed limit’ by itself. I have a formula that explains this ‘impossible speed’, but unless you understand what I am saying so far, it is useless for me to continue.

Cheers
Johannes