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Ultra Low Density Lipoproteins (ULDL), historically commonly called chylomicrons (dating to before these particles were better characterized and understood) are one of the five major groups of lipoprotein, as divided by density or size [an inherently inverse relationship because the larger particles (higher ratio of fat molecules inside compared with the outer emulsifying protein molecules in the shell) and fats are always lower density than either protein or water molecules, i.e. fats float in water.]. Specifically they are the largest and only lipoprotein particles (which transport fat molecules around the body in the water outside cells) which can sometimes (if large enough, at close to about 1,000 microns or more) be seen using only a light microscope, at maximum magnification.
All the other classes are submicroscopic, one of the reasons these particles were long difficult to identify and better understand.