The fact of Gravity is as obvious as an apple falling from a tree. The fact of Evolution is as obvious as the millions of bacteria reproducing all around you this very second.
The Theory of Gravity isn't as straightforward as one would like to think. I'm no expert on this, but I will give it a go. Gravity is one of the four known fundamental forces of the Universe. The other three are electro-magnetism, the strong nuclear force, and the weak nuclear force.
If I remember my General Relativity correctly, gravity is a weak force that exerts influence over very long distances. It is believed that matter distorts space, and this distortion is perceived as the attraction of gravity between objects. For example, most people assume that the Earth orbits the Sun because of the force of gravity exerted by the Sun over the Earth. It's a simple Newtonian view of the Universe. However, Einstein explained that in fact the Earth and Sun are both moving in "straight" lines. The mass of the Sun causes a curvature of space-time, therefore the Earth is moving in a straight line through curved space. The Sun travels through curved space around the galaxy, the mass of which, containing billions of stars, curves space, even influencing the paths of nearby galaxies in our Local Group.
The Theory of Gravity makes certain predictions which may not be directly observable to the lay person with little scientific training. These predictions may not be so obvious as the "fact" of an apple falling from a tree. In this way, the "Theory" of Gravity operates as a set of abstract concepts which allow certain predictions to be made. As the abstract concepts are refined and improved over time based on evidence and experiments, it is assumed that the "Theory" becomes more in line with reality. This is "rocket science" after all!
If you move to a new city and have to learn how to get around it, you develop a mental map of the city in your mind. The mental map is an abstraction of images, words, and numbers that allow you to make sense of the reality of the city itself. The city is a "fact." Your mental construct is your "theory" in a sense. As you get to know the city better, your "theory" becomes more in line with reality.
The fact that life is constantly evolving is explained by the Theory of Evolution. As with the Theory of Gravity, it is a set of abstract concepts which allow certain predictions to be made. It also makes certain predictions which may not be directly observable to the lay person with little scientific training. Part of the difficulty lay people have with the Theory of Evolution is that the biological changes it predicts require enormous amounts of time to occur. We don't observe major changes in higher order plants and animals occurring within mere decades. Evolution doesn't predict that kind of change. However, the evidence from genetics, the geological record, and the fossil record allow scientists of various disciplines to formulate theories about what happened in the distant past. So, there are several observable facts: the genes found in living things today which can be examined to find similarities and differences between living organisms; rocks which contain the remains of living beings from the past and which show changes in the forms of once living beings over periods of time; and the forces which shape and change the surface of planet Earth.
Evolution does not act on dead things. Evolution only occurs in the "present." Evolution is happening as I type these words. It is a fact. It is happening right now. Every living thing is playing its part in the ongoing story of life on this planet. To imagine that the Theory of Evolution is only about fossils, dinosaurs, and ape-men severely misses the point. Yes, due to the incredible amounts of time involved for major changes to occur, the Theory of Evolution appears to focus on the past, but I can assure you one thing: long after the Bible and Fundamentalist Christianity and the Watchtower Society have ceased to exist or even be remembered, life will still be evolving on the Earth.
