White at the bottom of the screen with ScrollView

I put ScrollView in one of my layouts , to allow users with smaller screens to see the whole information.

The problem is that when I start the application on my phone (5.5 '') I will see a white band above the layout :

Thecodexmloflayoutisasfollows:

<ScrollViewxmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:overScrollMode="ifContentScrolls"
    android:fillViewport="true"

    >
<RelativeLayout
    android:id="@+id/activity_main_apresentation"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorBackGround">


    <ImageView
        android:id="@+id/imagetoMain"
        android:layout_width="wrap_content"
        android:layout_height="400dp"
        android:contentDescription="ola"
        android:src="@drawable/cakelogomain"

        />


    <TextView
        android:id="@+id/txttoDescription"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/imagetoMain"
        android:layout_centerHorizontal="true"
        android:text="@string/description"
        android:textColor="@color/colorSecondBackGround"
        android:textSize="18sp"
        android:textStyle="bold" />


    <TextView
        android:id="@+id/texttoViewTodscr"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/txttoDescription"
        android:gravity="center_horizontal"
        android:text="@string/descriptionTwo"
        android:textColor="@color/colorSecondBackGround"
        android:textSize="10sp" />

    <Button
        android:id="@+id/idButton"
        android:layout_width="350dp"
        android:layout_height="wrap_content"
        android:layout_below="@id/texttoViewTodscr"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="10dp"
        android:background="@color/colorSecondBackGround"
        android:text="@string/botaoMainTransition"
        android:textColor="@color/colorWhite" />



</RelativeLayout>

</ScrollView>

Can anyone give me an explanation of why this happens?

Any better way to adapt my layouts to smaller screens?