<p>The detection of gravitational waves, carried out for the first time in 2015, seemed to decisively support a materialistic conception in the long-lasting controversy on the material nature of space–time. If gravitational waves interact with a detector and every interaction involves a transfer of energy, these waves—and the very space–time of which they are ripples—must necessarily be material as well. In this paper we will analyze the applicability to space–time of three interrelated criteria of materiality according to which matter is everything that (1) has energy, (2) can change and (3) can interact. It will be noted that these three criteria, although valid in the other cases, present serious problems when they are applied to space–time, considering it as a material entity. Such problems arise from the peculiar geometric interpretation of gravity proper to general relativity, which reinforces the possible exceptionality of gravitation among the fundamental interactions of nature. All this leads to consider the need to elaborate new ontologies that go beyond our present conception of matter, including space–time itself in this category, and thus overcoming the old debate between relationalism and substantivalism.</p>