Yes, something can exist that isn't matter. In addition to matter, the universe is also composed of energy, which can exist in various forms such as light, heat, or electromagnetic waves. Energy is not considered matter because it does not possess mass or occupy physical space in the same way that particles of matter do.
Moreover, there are abstract concepts and entities that exist but are not composed of matter or energy. For example, thoughts, ideas, mathematical principles, and logical concepts exist independently of physical matter. These abstract entities are often considered to exist in the realm of the mind or as conceptual constructs.
Furthermore, there are theoretical entities and phenomena in physics that are not directly observable but are believed to exist based on their effects. Examples include dark matter and dark energy, which are hypothesized to make up a significant portion of the universe's mass and energy but do not interact with ordinary matter and are not directly detectable with current technology.
So, while matter is a fundamental component of the universe, there are other forms of existence that go beyond it.