The Resource Market is a page found in the Merchant Tab, where you can call a resource trader in exchange for dark matter, like the commander / officers, or can be discovered on an expedition. When you call him, he comes to the planet you're on at the time (if you're on your home planet, he comes to the home planet; if you're on a colony, he comes to that colony). The trade-rate he uses is 3:2:1 (metal:crystal:deuterium). It is not a fixed rate, it can vary a bit. Every time the merchant is called, the rate is selected randomly.