By northern states, I mean states loyal to the United States of America during the Civil War. The answer is yes, there were slaves in the North. In fact, there were more than 450,000 in 1860. Eight northern states and Washington DC practiced slavery while the seven confederate states did so.
The reason was that the states each chose their path toward abolition, and each plan progressed at its own rate. It was much easier for many northern states because their land did not lend itself to plantations, and by the time of the Civil War, all tobacco and cotton plantations had moved to the south. The North could deal with the economic effects much easier than the South. Still, even while progressing on a plan, some states, such as Delaware, had not legally declared abolition of their slaves.
The South knew that abolition was coming, but its path was much more difficult economically. Still, the number of plantation owners was quite small compared to the overall population—but those were the people in the legislature.
As the South continued to look for a path out of slavery, the Civil War erupted and slavery was, of necessity, tabled because of the overall need for defense measures.
Meanwhile, the North’s virtue signaling intensified through the Abolitionists who did not take the North-South differences into consideration and demanded immediate abolition everywhere, although the South was elected to play the bogey-man.