HERE IS THE ANSWER
This question had confounded the scientific community for a very long time (it still does to a large extent). The most plausible answer can be given in terms of what we called "Higgs field"
The Higgs Field is an energy field that exists everywhere in the universe. The field is accompanied by a fundamental particle called the Higgs Boson, which the field uses to continuously interact with other particles. As particles pass through the field they are "given" mass, much as an object passing through treacle (or molasses) will become slower.
Mass itself is not generated by the Higgs field- the creation of matter or energy would conflict with the laws of conservation. However, mass is "imparted" to particles from the Higgs field, which contains the relative mass in the form of energy. Once the field has endowed a formerly massless particle the particle slows down because it has become heavier.
If the Higgs field did not exist, particles would not have the mass required to attract one another, and would float around freely at light speed.
The process of giving a particle mass is known as the Higgs Effect.
(the theoretical Higgs Field does too)
(the theoretical Higgs Boson gives energy mass)
Sincerely yours in Jesus through Mary,
Become like Jesus
Live for the Triune God
Seek the Light of Our Lord Jesus Christ
See you on the High Ground!
* - J.M.J. + O.B.T. + M.G.R. stands for:
Jesus, Mary and Joseph;
O Beata Trinitas;
St. Michael, St. Gabriel and St. Raphael