Matter is composed of particles and that particles are called atoms.
since the atom is very small as cannot be seen by naked eye the particle will also be microscopic in nature.Atoms can be seen only through a Scanning Tunneling Microscope.
It's the property of matter of particle to be microscopic.
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Matter is made up of tiny microscopic particles called atoms and the size of an  atom is so small that it cannot be seen through a naked eye therefore  the particle are microscopic in nature.therefore particles of matter are microscopic in nature
Gases, liquids and solids are all made up of microscopic particles but the bonding is different and they behave differently than each other.
as the size of atoms are small the 
particles of matter are microscopic in nature