“The market doesn’t like unknowns, and the situation in Greece today was a definite unknown,” Stephen Solaka, co-founder of Belmont Capital Group in Los Angeles, said in a Bloomberg report. “Bad news is OK as long it’s known. Unknown news or surprises cause issues, cause sell-offs and fears to appear. Fear is back on the table.”

The Greek referendum is viewed by market watchers as an event that could stir more financial instability and dampen the outlook for the Eurozone.

iPath S&P 500 VIX Short-Term

Tisha Guerrero contributed to this article.