Modern science is based on the findings is experiments conducted according to the scientific method. Observations made by science are trusted because they are the result of unbiased data that can be replicated anywhere with the right equipment. Modern science is not based on assumptions. You can assume that God exists, or that the universe was created 6,000 years ago, or that a younger universe wouldn't need dark matter to have some of the the physical characters that ours does (I think that's what your getting at?) None of these assumptions mean anything because they are not accepted as scientific fact, and at least one of your assumptions (the 6,000 year thing) has been definitely disproven dozens of times by various scientific fields. I could assume that God is real and gives us all challenges to overcome, and that He guided you to make this post because He wanted you to understand creation through a scientific lens rather than a scriptural point of view. Judging by your edit that's asking too much.