Antimatter is the opposite of matter, dark energy is a theory to explain the Universe expansion.
Normal matter is made up of particles. An atom has a nucleus containing protons and neutrons. It also has shells of electrons around the nucleus.
Most particles have anti particles. There is an antiproton, and antineutron and a positron (anti-electron).
If you were to assemble atoms from these anti-particles you have antimatter. If particles and their antiparticles meets they annihilate each other to form photons.
When the Universe was formed there should have been equal amount of matter and antimatter. Some unknown cause broke the symmetry so that there was more matter than antimatter, so only matter is left. Antimatter has been made in small quantities in laboratories.
It has been observed that the Universe is expanding at a rate faster than theories allow. A theory has been developed which requires a yet unknown quantity called dark energy to permeate space and cause the Universe to expand at an accelerated rate.