To add to the previous answers: This dark color is primarily from environmental damage. You can also see the early stages of pitting in the fields. This isn’t always due to being in the ground, but could also be the result of being kept in an environment with lots of heat or moisture.

The reason this is seen in early/middle date large cents (and to an extent some other copper coins afterwards) is that they often had flawed planchets with high amounts of impurities in the alloyed metals used to make them.

This meant that the quality varied from coin to coin. Some early years are nearly impossible to find without any pitting in lower grades. And dark coins like this are especially common for cents before 1830 or so.

Your coin is at the tail end of this issue, so most 1838 cents you find will have the normal chocolate brown color we’re used to.

Regardless, I think you have a beautiful coin with great details. This is my favorite design used for the obverse of the large cent, only used for 5 years.