A "complex mixture" refers to a substance made up of multiple different components mixed together. It is called "complex" because it is not made of just one thing, but rather a combination of many different things all mixed up. Full definition
Space also holds a more complex mixture of particles than those in the test beam. (sciencenewsforstudents.org)
«We expected some of the species from the more complex mixture to disappear over time,» he explains. (sciencedaily.com)
So depending on the regulation of protein expression, the feathers that the proteins constitute must be made up of a very complex mixture of building blocks. (sciencedaily.com)