A close-up

Close up of the cracks

"I understand there are always gaps in between the flooring"

Your understanding is incorrect? This was unfinished flooring and then the finish applied on site? What type of finish was applied? HVAC up and running well before flooring brought into the home to acclimate? Readings taken of the subfloor and the flooring before installation with the readings to the flooring manufacturers range limit?

The floors were unfinished and stained on site with, I believe, duraseal. Hvac was running.

What was put on top of the stain? How long did the flooring acclimate? Was the subfloor and the flooring before install measured for the moisture percentage? Do you have the paperwork with this on it?

The cracks aren't jagged. What you're seeing is the remainder of filler that was applied. As the wood shrank, some stuck to one board while some stuck to the adjacent board.

Millworkman's questions are pertinent. There are quite a few prior posts on this well known phenomenon. Search "seasonal gapping" in Flooring. You'll need to research all the things listed above and it would be ideal to purchase a thermo-hygrometer to track the humidity inside your home.

What is your current outdoor temperature and humidity?

Increase the moisture in the air. The gaps will close up, but may take a few weeks.

As above. The jagged edges are due to DRY air causing wood to shrink SO fast the finish has split and cracked. You MUST increase your HUMIDITY in the home. You need it to be/stay at 45% or so.

Your HVAC is one thing...your whole home humidifier is another.

First things first...what is your indoor humidity? Now you need to find out if you have a whole home humidifier. If you don'tyou need to upgrade your system. That means more money.

Current temperature is in the 30-40’s. We live in NJ. Is it normal to use wood filler throughout the entire house? These jagged edges are on about 25% of the flooring. I’ve never seen this before and never had to install a humidifier in prior houses.

Wood filler on floors is hillbilly.

Is that INDOOR temp???? Is that 30-40 F??? Or is that 30-40 C? (yes I've seen people pump the heat to 35+ Celsius).

It doesn't matter if you did not 'need' a humidifier in a different house. This is your current home...and you need one. Trust me...you NEED IT!

Or you can expect these horrible jagged edges showing up every winter...and each time it happens you will have POUNDS of wood filler crumbling out or ripping your socks each and every year...until you get fed up and either rip out the floors (shame...don't do it) or you add the WHOLE HOME humidifier.

You've dropped some SERIOUS cash with these floors. A whole home humidifier is the last step (actually it's the first step...but better late then never). If someone told you $1K - $2K could offer 'wood flooring insurance' would you purchase it? Many people would. Those are the people who go ahead and add indoor humidity.

For some, that means a portable humidifier ($500 - $900 unit) to get them through the first winter while they save up for the BIG upgrade (depends on the size of the home and the age of the HVAC system).

It's your floor. You JUST got it. How long would you like it to last? It's your call.

But first things first...find out how DRY the house is...purchase a hygrometer at Home Depot for $35.

Outside temp 30-40 Fahrenheit indoor about 65. Thank you for the advice! Will do immediately

@Joseph Corlett, LLC

I don't know how much of your sink-replacement business involves installation and finishing of hardwood floors, but using filler prior to sanding hardwood floors that are finished in place is typical--not "hillbilly" as you've characterized it. But don't take my word on it:

https://hardwoodfloorsmag.com/2017/10/02/fundamentals-sanding-wood-floors-part-4-filling-fine-sanding/

Charles, you are correct and I thank you for the link. I should be more specific when I post. Look at the floor pictured in your link please. You aren't getting much if any filler between those floorboards. Attempting to fill the Grand Canyons pictured in this thread is hillbilly. I'll attempt to be more clear in the future.