Yes, dark matter is currently considered necessary to explain the observed rotational speed of galaxies, including the Milky Way. The rotational speed of stars and gas within galaxies can be measured by studying their spectra and observing their Doppler shifts. Based on these observations, it was initially expected that the rotational speed would decrease as one moves farther from the galactic center, similar to how the gravitational force decreases with distance in classical mechanics.
However, observations have shown that the rotational curves of galaxies, including the Milky Way, remain relatively flat, indicating that stars and gas are moving at high speeds even in the outer regions of galaxies where the visible matter is less dense. This implies the presence of additional mass that cannot be accounted for by the visible matter (stars, gas, and dust) alone. This is where the concept of dark matter comes into play.
Dark matter is a hypothetical form of matter that does not interact with light or electromagnetic radiation, hence its name "dark." It is believed to be present throughout the universe, including galaxies like the Milky Way. Its gravitational influence provides the additional mass needed to explain the observed rotational speeds and the stability of galaxies.
Various lines of evidence, such as gravitational lensing, the motion of galaxy clusters, and the cosmic microwave background, support the existence of dark matter. However, the exact nature of dark matter remains unknown. Scientists are actively studying and conducting experiments to better understand its properties and composition.
It's worth noting that alternative theories have been proposed to explain the observed galactic rotation curves without invoking dark matter, such as modifications to the laws of gravity (e.g., Modified Newtonian Dynamics or MOND). However, currently, the majority of astrophysicists and cosmologists find that dark matter provides the most consistent and comprehensive explanation for the observed phenomena in galaxies.