expansion of the market niche

'market niche' is a selling idiom.

It speaks to an audience who will most likely buy your product. For example, Skechers Shape-Up sneakers would be most likely bought by people under 30 or young mothers. The company would now hope to sell that product to people over 30, so they might change the product slightly so that market niche would be larger.

"market" in this sense is a term used in business to signify that the specific demograpahic of people who tend to use/prefer/buy products and services the company sells and "targets" them through advertising methods to increase company profitability by making the most use of their advertising dollar.

Broadening or expanding the market means the company is looking to pull in a higher percentage of customers from the population

. . . and 'niche' means something that fits just right.

Yes. A small corner of a much larger market. A good niche can yield a high profit from a small section of a market.

it seems to me market niche = marketing strategies.

so , company is expanding their marketing strategies to pull more customer.
I think , I 'm correct now.