Speed of light described as the speed of light 299792458 m/s in vacuum that is in the absence of any particles, fields and any medium.
But in universe we never achieved a place with perfect vacuum. Only true massless particles likes photons and gravitational waves and gluons can travel with speed of light.
Any particles other than that in the universe like dark matter, neutrinos, leptons, quarks all have the masses and can't achieve speed of light.
Our universe don't have any vacuum place. All space either covered with CMB( particles with Cosmic microwave background), WHIM particles (Warm hot intergalactic medium) CNB particles (Particles from cosmic Neutrino Background)
That's why scientists are telling that there is a speed limit to the particles that travel through the universe and it's not the speed of light. It's value lower than speed of light.
As our universe is continuously expanding and cool and for the same reason it's speed limit also slightly lower than speed of light.