No, dark matter was not explicitly a part of Einstein's theory of general relativity. Einstein's theory of general relativity, formulated in 1915, primarily focused on the description of gravity as the curvature of spacetime caused by matter and energy.
Dark matter, on the other hand, is a concept that emerged much later, in the 20th century, as scientists observed discrepancies between the observed gravitational effects in galaxies and the visible matter present within them. Dark matter refers to an elusive form of matter that does not interact with light or other electromagnetic radiation, making it difficult to detect directly.
While Einstein's theory of general relativity provides a framework to describe the behavior of gravitational interactions, it does not explicitly explain the nature or existence of dark matter. The presence of dark matter in the universe was inferred based on its gravitational effects on visible matter and the large-scale structure of the cosmos.
Explaining the nature of dark matter falls within the realm of modern physics and cosmology, and various hypotheses and theories have been proposed to account for it. These theories go beyond the scope of Einstein's original theory of general relativity.