Simplify the following text for a Grade 8 native English-speaking person.
Original: According to the prevailing scientific model of the universe , known as the Big Bang , the universe expanded from an extremely hot , dense phase called the Planck epoch , in which all the matter and energy of the observable universe was concentrated .
Simplified: The most used scientific model of the Universe is known as the Big Bang theory . The Universe expanded from a very hot , dense phase called the Planck epoch , in which all the matter and energy of the Universe was concentrated .