There are many theories regarding the nature of dark matter, but one thing is clear: it's physical properties are very different to those of "normal" baryonic matter. For one thing it only interacts with baryonic matter thorough gravity and the weak nuclear force. It's lack of interaction thorough electro-magnetism is presumably the reason it cannot be seen (optical light is composed of photons, the "carrier particles" of the electro-magnetic force).
Dark matter was discovered through observation of the rotation of distant galaxies, through the Doppler effect. The light of an object moving towards us will be shifted to the higher, blue, end of the spectrum. The light of an object moving away to the red. Though due to the expansion of the universe the center of galaxies will always be shifted to the red, the rotation will cause on end of it to shift to the red and the other to blue. The extent of the shift allows us to measure the velocity of the rotation. Upon examining the the date it became apparent that the visible matter be solely responsible for holding the galaxies together - the gravity they create is not strong enough.
Scientists currently estimate dark matter to make up around 90% of all matter. That underlines the difference between dark matter and luminous matter- it indicate that dark matter cannot be simply non luminous objects in the examined galaxies, objects that produce little or no light and therefore cannot be observed at a distance: planets, brown dwarfs, neutron stars, black holes ect. They certainly contribute to the overall gravitational force, but even if we assume them to be as massive as all the luminous matter (unlikely as most non luminous objects are significantly less massive), we still would not come close to making up the "missing" 90%. It mast be a substance of a different Nature.
Dark matter is all around us, though recent studies suggest that there are greater concentration of dark matter where no luminous matter is present - in the voids between great galaxy clusters. Dark matter is a wonderful example of the universe revealing itself in new and wonderful ways. The more we know, the deeper and further our scientific knowledge the more new mysteries yet to be understood we discover.