Solid Hardwood, Engineered and Laminate Flooring - hardwood floor gaps

My house is 2 years old, I have red oak hardwood in most of the areas, this was unfinished boards which were finished at the site. I have gaps between the boards, most of them can pass a dime, few of them can even pass a quarter, of course they get worse in the winter time, we live in pennsylvania, but you still can see them in the summer. Is this normal, and is there anything I can do . somebody told me I can put a wood filler. I really don't know how good this idea is.

Most tend not to recommend filler in gaps between boards, as the expansion and contraction forces the filler out. Maintaining level of household humidity at between 35-55% year round tends to lessen the effects of expansion and contraction.

Fill them when they are the smallest, during the summer.

It sounds like a poor install. I don't think there is really much you can do to fix it short of yanking it up and re-installing.