It was previously believed that all matter in the Universe interacted with its surroundings both gravitationally (the bending of space-time) and electromagnetically. Researchers later discovered that when the mass of cosmological objects was measured gravitationally, the result was different than if it were measured based on visible matter. They concluded that there must be another type of matter which can only be observed through gravitational effects, and called it dark matter. The concept of God is so variant and open to interpretation that it is difficult to find something that would not be considered God somewhere, by someone. If by God you mean an ever-present material that has an impact on everything in the universe, then dark matter could certainly work. I would consider myself an atheist because I reject the idea of a God as a creator with purpose. But there is still so much about the universe that we don't know, the truth is, anything is possible.