Whenever these are an alarming due to a foreign antigen, the Helper T cells, Memory T cells, and Cytotoxic T cells are being produced. Helper T cells to recognize the specific foreign antigen that is entering the body, Memory T cells to remember the specific foreign antigen that is attacking, and Cytotoxic T cells that are for the destruction of the foreign cells and produce cytokines to attract macrophages
The main reason for this is gravity. Gravity between objects in our solar system and milky way is strong enough to counteract this expansion. Hope it helps.