Another Answer to a Yahoo answers question too long for yahoo answers.

http://answers.yahoo.com/question/index;_ylt=Am_KvU1lyVWBPbWdSSK16P8jzKIX;_ylv=3?qid=20080905033254AACHBs4

Above is the question, below is my answer in full.

They are scientific theories thus many aspects of the theories are facts and the theories themselves are based on facts. The theory of evolution, for instance, is based on the fact of evolution. Evolution is simply defined as descent with modification. This basically means that every succeeding generation will not be a perfect genetic carbon copy of the preceding generation. These modifications can be neutral, beneficial or detrimental. The neutral modifications will have no noticeable effect or a bad or good effect depending on how the environment changes and only if the environment changes. Beneficial modifications will have a positive effect even if the environment stays the same. Detrimental mutations have a negative effect even if the environment stays the same.
Positive mutations are more likely to be selected for by natural selection because they will confer a survival advantage on the organism with the mutation making it more likely that the individual will survive and reproduce. When that individual reproduces it will pass on its beneficial mutation to its offspring. The theory of evolution suggests, with evidence, that eventually as these mutations accumulate new species can emerge even differentiating into new genuses and families, etc. You should try reading Darwin's The Origin of Species.
The evidence of this comes from fields that were already existent durring Darwin's lifetime and fields that were not yet existent durring Darwin's lifetime. There's paleontology. The fossil record confirms the evolution of, just to name a few examples, amphibians and reptiles from fish, the evolution of mammals from reptiles, the evolution of birds from reptiles, and the evolution of aquatic mammals from land mammals. It even shows a clear evolutionary pathway for specific species such as horses, elephants, pigs, humans, etc. The evolutionary theoy is also confirmed by genetics. A prediction of the theory of evolution for genetics would be that if the theory were correct you would expect to see a direct correlation between how closely related two species are and how similar their genomes are. This is exactly what we do, in fact, see. This is also only two examples of the evidence for evolution, there is much much more. I gave links.
Likewise the big bang is based on observations of the real world, for instance the fact that the universe is expanding. This is observed in the redshift of galaxies. Before this theory scientists believed that the universe was eternal and static, this model is known as the steady state model. Of course Einstein's theory of general relativity was the final nail in the coffin of the steady state model, even though there are still defenders of this now dead model. If gravity works the way that Einstein described, and countless experiments have confirmed that it does at least above the quantum scale, then there would have to be some special voodoo at work in the universe to keep the universe from collapsing in on itself. Of course if the universe is still expanding outward because of the force of what was initially thought of as a big bang that would explain why the universe isn't collapsing in on itself, in other words why all the galaxies aren't blue shifted. There's another reason why the theory of relativity marks the death of the steady state model, though, which Einstein himself advocated. Einstein's equations, taking the observed universe into account, when run backwards 13.7 billion years, gives us a big crunch. This would be the big bang in reverse. The interesting thing is that Einstein developed his equations with the assumption that the universe was eternal and static, he absolutely hated the idea that there could have been a beginning to the universe. But despite that his equations clearly show that the universe had a beginning which is referred to as the big bang.
Of course at the end of relativity's big crunch is a singularity. Because relativity assumes spacetime is continuous it crunches the universe down to infinite density, at which point the equations break down creating a singularity, which would be the equivilent of division by 0. The singularity is really just the point where the math becomes undefined. The common belief among cosmologists is that if we are going to understand the big bang we are going to need a quantum theory of gravity. One that brings quantum mechanics and relativity together.
One candidate is Loop Quantum Gravity, and you may be interested to hear that Loop Quantum Gravity predicts a model that changes the big bang to the big bounce. The mathematics of the theory only assumes the existence of spacetime atoms, which makes space not continuous and allows the math to be defined down to and before the moment of the big bang, allowing us to be able to understand what happened. Loop Quantum Gravity is still incomplete, but it's current state of readiness suggests that at extreme density (albiet finite density) the gravitational force changed from a weak attractive force to a very strong repulsive force causing rapid spacetime expansion as predicted by the inflationary model of the big bang. I'll provide a link to the article about the big bounce from scientific american as well.

http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_01http://www.gate.net/~rwms/EvoEvidence.htmlhttp://anthro.palomar.edu/evolve/evolve_3.htmhttp://www.talkorigins.org/faqs/comdesc/http://www.talkorigins.org/origins/faqs-evolution.html
http://www.sciam.com/article.cfm?id=big-bang-or-big-bounce