為何人知道太空是真空?

Space is NOT really empty. There are lots of atoms, molecules and energy (photons) in space. These substances are collectively known as "interstellar matters"(星際物質).

Among the interstellar matters, hydrongen atoms are most abundant. The concentration of which is measured by their radio emission when they are bambarded by cosmic rays (i.e. extreme high energy protons).

因為萬有引力可以令到所有鄰近的物件飛唔起，或者係 (空中各種粒子) 飛唔出一定的距離，咁過左個一個距離，咪應該係真空囉。