Eternal does not mean existing forever, at least not necessarily. It does mean existing outside of time. That is another reason we can know that matter has not existed for eternity because matter and energy cannot precede themselves. They cannot precede themselves because “Coming Into Being” is an essential and objective feature of time.
Time, Space, Energy, Matter and the Laws of Physics that govern the constants and qualitities that were in place prior to Planck time came into existence at the Big Bang.