Particles in quantum mechanics have the property of quantum wave entropy. Similar to stretching a spring, under the specific conditions of cosmic expansion, particle wave entropy exhibits a negative entropy force that resists expansion. This negative entropy force provides particles with an attractive effect different from gravity. Like a spring constant, the negative entropy force has a proportional factor. The negative entropy force is proportional to the Hubble constant and also proportional to gravitational rotational speed. The effect of this negative entropy force is roughly the same as that of dark matter, serving as the physical origin of dark matter effects. Negative entropy force leads to a result where the rotation speed of a galaxy is proportional to the fourth root of the distance, showing an approximately flat and slowly rising curve. Negative entropy force also can explain the Tully-Fisher relation. Galaxies have properties of a flat radius and a cutoff radius. Galaxies only show noticeable dark matter effects within a limited range. The dark matter effect becomes noticeable starting from the flat radius and begins to decline noticeably from the cutoff radius. The flat radius and cutoff radius of a galaxy are determined by its mass. Galaxies of different masses and sizes will show different behaviors of dark matter. Negative entropy force also applies to photons and can explain the dark matter effects in gravitational lensing. Therefore, without assuming the existence of dark matter or modifying gravity theory, we can still normally explain the observed dark matter phenomena in galaxies. Dark matter doesn't exist, and MOND isn't needed.