Time is not an absolute measure but is different for everyone becasue time is affected by gravity. The stronger gravity is the slower time goes. The difference is very small, a clock on the suns surface would go only about one minute slower than one on the earths surface. Our biological clocks are also affected by gravity. Dark energy was postulated to account for tiny ripples detected in bacround microwave radiation. The ripples indicated that the universe might be flat, the third Friedmann model. There was not enough matter or dark matter to account for this so they said that an undetected substance called dark energy might exist. Dark energy has never been observed while dark matter has.
Something else that has not been mention in detail here is anti-particles, which basically are what anti-mater is made of. An example is the electron, which has an anti-particle the positron. Particles have the same mass as their anti-particles but are opposite in charge and other attributes. A positron has a positive charge, the opposite of an electron. If a positron and electron ever touched they would annihilate each other, creating photons. However to create a particle/anti-particle pair you have to make two massless particles, such as photons, collide with a certain amount of energy so that the mass can be created.
