Brian Koberlein: "This led Henri PoincarĂ© to propose non-electromagnetic stresses to hold the electron together. When he calculated the energy of these stresses, he found it amounted to a fourth of an electron's total mass. Thus, the "actual" mass of the electron due to its electric charge alone must be m = E/c^2. PoincarĂ©'s paper deriving this result was published in June of 1905, just a few months before Einstein's paper. Although the equation is often attributed to Einstein's 1905 paper, Einstein didn't actually derive the equation from his theory of relativity."

https://www.forbes.com/sites/briankoberlein/2017/11/09/the-history-of-einsteins-most-famous-equation/
Hans C. Ohanian: "Although Einstein's name is closely linked with the celebrated relation E = mc^2 between mass and energy, a critical examination of the more than half dozen "proofs" of this relation that Einstein produced over a span of forty years reveals that all these proofs suffer from mistakes. Einstein introduced unjustified assumptions, committed fatal errors in logic, or adopted low-speed, restrictive approximations. He never succeeded in producing a valid general proof applicable to a realistic system with arbitrarily large internal speeds."

https://arxiv.org/ftp/arxiv/papers/0805/0805.1400.pdf
Philip Ball: "Did Einstein discover E = mc^2? Who discovered that E = mc^2? It's not as easy a question as you might think. Scientists ranging from James Clerk Maxwell and Max von Laue to a string of now-obscure early 20th-century physicists have been proposed as the true discovers of the mass-energy equivalence now popularly credited to Einstein's theory of special relativity. These claims have spawned headlines accusing Einstein of plagiarism, but many are spurious or barely supported. Yet two physicists have now shown that Einstein's famous formula does have a complicated and somewhat ambiguous genesis - which has little to do with relativity."

http://physicsworld.com/cws/article/news/46941
Philip Ball: "The biggest revelation for me was not so much seeing that there were several well-founded precursors for the equivalence of mass and energy, but finding that this equivalence seems to have virtually nothing to do with special relativity. Tony Rothman said to me that "I've long maintained that the conventional history of science, as presented in the media, textbooks and by the stories scientists tell themselves is basically a collection of fairy tales." I'd concur with that."

http://philipball.blogspot.com/2011/08/did-einstein-discover-emc2.html
Lisa Randall, Michio Kaku, Brian Cox, Neil deGrasse Tyson, Brian Greene: "Why do the stars shine? Why does the galaxy light up? E equals MC squared. That is the engine that lights up the stars. Energy turns into mass. E equals MC squared - that is the secret of the stars. Now, listen carefully. The faster you move, the heavier you get. Light travels at the same speed no matter how you look at it. No matter how I move relative to you light travels at the same speed. No matter who is doing the measurement and no matter what direction you are moving the speed of light is the same. The speed of light is the same no matter what direction or how fast... As you travel faster time slows down. Everything slows down. Everything slows down. Time slows down when you move. Time passes at a different rate. Clocks run slow. It's a monumental shift in how we see the world. It's a beautiful piece of science. It's a beautifully elegant theory. It's a beautiful piece of science. It's a beautiful piece..."

https://www.youtube.com/watch?v=BuxFXHircaI
We all live in Einstein's schizophrenic world, don't we?

Pentcho Valev