it is true that matter and energy are just two sides of the same coin. In fact, you've known this your whole life, if you've ever heard of the formula E=mc^2. The E equals Energy and the m equals Mass. The amount of energy contained in a particular amount of mass is determined by the conversion factor c squared, where c represents... the speed of light.
The explanation for this phenomenon is really quite fascinating, and it has to do with the fact that the mass of an object increases as it approaches the speed of light (even as time is slowing down). It is, however quite complicated, so for the purpose of this blog post, I'll simply assure you that it's true. For proof(unfortunately), look no further than atomic bombs, which convert very small amount of matter into very large amounts of energy.