What causes the observed dipole anisotropy in the CMB temperature measurements?

The motion of Earth and the solar system relative to the cosmic rest frame.

The dipole anisotropy observed in the Cosmic Microwave Background (CMB) is not a reflection of a true structural imbalance in the universe itself, but rather a kinematic effect caused by the observer's movement. Because the Earth and our solar system are moving relative to the cosmic rest frame established by the CMB, the radiation appears slightly altered based on direction. In the direction of motion, the radiation is blueshifted, making it appear slightly hotter. Conversely, in the opposite direction, the radiation is redshifted, making it appear slightly colder. Once this local motion is accurately calculated and removed, the remaining background radiation confirms the underlying isotropy with high precision.