Computer science is often said to be neither a science nor about computers. There is certainly some truth to this claim–computers are merely the device upon which the complex and beautiful ideas in computer science are tested and implemented. And it is hardly a science of discovery, as might be physics or biology, so much as it is a discipline of mathematics or engineering. But this all depends on which branch of computer science you are involved in, and there are many: theory, hardware, networking, graphics, programming languages, software engineering, systems, and of course, AI.
Computer science theory is often highly mathematical, concerning itself with questions about the limits of computation. Some of the major results in CS theory include what can be computed and how fast certain problems can be solved. Some things are simply imposible to figure out! Other things are merely difficult, meaning they take a long time. The long-standing question of whether “P=NP” lies in the realm of theory.
A subsection of theory is algorithm development. For instance, theorists might work to develop better algorithms for graph coloring, and theorists have been involved in improving algorithms used by the human genome project to produce faster algorithms for predicting DNA similarity.
Cryptography is another booming area of the theory section of computer science, with applications from e-commerce to privacy and data security. This work usually involves higher-level mathematics, including number theory. Even given all of the work in the field, algorithms such as RSA encryption have yet to be proven totally secure.
Work in theory even includes some aspects of machine learning, including developing new and better learning algorithms and coming up with bounds on what can be learned and under what conditions.