Hello friends, I want to precisely understand what dark matter is. Is Dark Matter a matter that is not visible but have gravitational effects? Why is Dark Matter not visible? Can Dark Matter be really big? I mean can it have considerable amount of mass or does it have negative mass? And I have Heard of Antimatter that it can annihilate energy when in contact with matter.But Why do they annihilate energy on contact? Since Antimatter is opposite of Matter,Does it have negative mass?