How do you believe dark matter & dark energy?

I myself don't think dark matter exists. I heard about antimatter that is believable and dark energy sometimes called as negative energy. Negative energy idea is just an unstablished theory & created to support higgs field where cancelling so called positive energy. Otherwise it contradicts relativity of E=MC2. Energy is just energy. Is it not the gravity that slows down the expansion? Then why quantum field is incomplete about the gravity but can be negatively charged but why dark energy?