If https://duckduckgo.com/ webpage has become browser’s default search engine without your consent, well then you picked up malware. At first glance, there is nothing suspicions because it looks like other search engines like Google or Bing. Nevertheless, presence of DuckDuckGo greatly reduces browsing experience as well as computer performance.
If you are seeing a lot of ads labeled as “Ads by GearBest” then your system is infected with adware. It could get into your system without your noticing through bundling method. Usually in such cases it infiltrates the system using other software when the users forget to opt out from unnecessary programs.