A Planck length is the smallest length the Universe can have. There is no smaller length than that and it is 1.161*10^-35 or about 10^20 times smaller than the diameter of an atom (1 femtometer, or 1 fermi or 10^-15 meters). When Big Bang expanded in the first second it expanded at a rate way bigger than the speed of light and even if you use Planck time, 5.3*10^-44 seconds, to measure this expansion, the expansion took more Planck times than there are seconds from the Big Band until now.
Now, that we learned a bit about these Plank things, there is also a Planck temperature, the hottest the Universe could ever get. If you set the light wavelength to the Planck length of about 1.616*10^-35 meters then you get a frequency of 1.85*10^43 Hz (based on formula: lambda[wavelength] * frequency = light speed) which corresponds to an energy of about 7.67*10^37 eV (eV is electron-volt, unit of energy, formula is X eV = 1240/ lambda ) and which corresponds to a temperature of 8.9* 10^41 degrees Kelvin ( formula: 7.67*10^37 eV = X K/ 11604 ). Check out this energy-wavelength calculator.
In any case, that is an EXTREME temp to say the least. Such energies would simply break all possible bonds and even let quarks loose like crazy. The weak nuclear force between nucleons is about 2 MeV and there are about 2000 GeV between the quarks that keep the protons together.
A temp of about 8.9*10^41 degrees Kelvin would simply create a quark soup where no atom could ever exist. Even though the inside of a black hole is cold, zero absolute K, the matter at the even horizon heats up to hundreds of millions of degrees, which is still nothing compared to the Planck Temperature. For now, no such object has been seen in the Universe we know.
Now, let’s let Vsauce explain these hotter than hot things like the Planck temperature: