Question: We all know how an electric current causes magnetic fields. But when a single electron is flowing through the wire, how is this electron creating the magnetic field or how are electrons responsible for magnetic field around a wire?

Answer: 
The ratio of dark to baryonic matter is 5.25 in the first diagram and 5 in the second diagram, but I don't think the difference is significant. We don't know the densities with absolute certainty, especially near the Big Bang, and the small difference between the ratios is probably just down to the uncertainties in the densities.

We would expect the ratio to stay fixed because the densities both scale as $a^{-3}$, where $a$ is the [scale factor](http://en.wikipedia.org/wiki/Scale_factor_(cosmology)). For comparison the density of relativistic matter (photons and neutrinos) scales as $a^{-4}$ and density of dark energy stays constant i.e. it doesn't depend on $a$ at all. That's why the photons and neutrinos have disappeared in the second diagram while a large chunk of dark energy has appeared.

As far as I know there is no mechanism for dark and baryonic matter to interconvert, or for either to disappear.
The following graph indicates the amount of radiation, matter (which almost is entirely made of dark matter), and dark energy vs time. We are at a point in time where dark energy has just started to dominate. For instance, if you go back to the first few years, radiation was the most abundant source of energy.

![Density vs time](https://i.stack.imgur.com/MXiov.gif)